Gray scale all-glass photomasks

ABSTRACT

A narrowly defined range of zinc silicate glass compositions is found to produce High Energy Beam Sensitive-glass (HEBS-glass) that possesses the essential properties of a true gray level mask which is necessary for the fabrication of general three dimensional microstructures with one optical exposure in a conventional photolithographic process. The essential properties are (1) A mask pattern or image is grainiless even when observed under optical microscope at 1000× or at higher magnifications. (2) The HEBS-glass is insensitive and/or inert to photons in the spectral ranges employed in photolithographic processes, and is also insensitive and/or inert to visible spectral range of light so that a HEBS-glass mask blank and a HEBS-glass mask are permanently stable under room lighting conditions. (3) The HEBS-glass is sufficiently sensitive to electron beam exposure, so that the cost of making a mask using an e-beam writer is affordable for at least certain applications. (4) The e-beam induced optical density is a unique function of, and is a very reproducible function of electron dosages for one or more combinations of the parameters of an e-beam writer. The parameters of e-beam writers include beam acceleration voltage, beam current, beam spot size, addressing grid size and number of retraces.  
     A method of fabricating three-dimensional microstructures using HEBS-glass gray scale photomask for three dimensional profiling of photoresist and reproducing the photoresist replica in the substrate with the existing microfabrication methods normally used for the production of microelectronics is described.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation-in-part of pending U.S. patent applicationSer. No. 09/507,039 filed Feb. 18, 2000 which is a continuation-in-partof pending U.S. patent application Ser. No. 08/961,459, filed Oct. 30,1997, which is a continuation in part of U.S. Provisional PatentApplication No. 60/030,258, filed Oct. 31, 1996.

BACKGROUND OF THE INVENTION

[0002] “High efficiency diffractive coupling lenses by three-dimensionalprofiling with electron lithography and reactive ion etching” by A.Stemmer et al, J. Vac. Sci. Technol. B 12 (6), November/December 1994,teaches three dimensional profiling of a photoresist and transferringthe three-dimensional microstructures of photoresist into the substrateusing reactive ion etching. Three-dimensional profiling of photoresistwith electron beam direct write on photoresist however is not costeffective for production quantities.

[0003] “Fabrication of diffractive optical elements using a singleoptical exposure with a gray level mask” Walter Daschner, et al, J. Vac.Sci. Technol. B 13 (6), November/December 1995 teaches generating a graylevel mask with eight discrete gray levels by means of cycles ofevaporation of Iconel and a following lift-off step. This gray levelmask allowed to expose a multi-level DOE in a single optical exposurestep for three-dimensional profiling of photoresist. CAIBE was used totransfer the analog resist structure into the substrate. The tightthickness control necessary in the Iconel evaporation steps makes thismethod of fabricating the gray level mask economically undesirable.

[0004] “Gray scale microfabrication for integrated optical devices”George Gal et al, U.S. Pat. No. 5,480,764, Jan. 2, 1996 teaches thefabrication of three-dimensional microstructres including photonicwaveguide surface, lens surface, and inclined planar surface for use asa beam splitter in a photonic device, using a half tone gray scalephotomask for three-dimensional profiling of photoresist and reproducingthe photoresist replica in the substrate with differential ion milling.However, a half tone gray scale mask is not desirable due to limitedresolution.

[0005] Other gray level mask fabrication methods have been demonstratedand show potential for mass fabrication. See for example: H. Andersson,M. Ekberg, S. Hard, S. Jacobson, M. Larsson, and T. Nilsson, Appl. Opt.29, 4259, 1990; and Y. Oppliger, P. Sixt, J. M Mayor, P. Regnault, andG. Voirin, Microelectron. Eng. 23, 449, 1994. After the mask fabricationonly a single-exposure step is necessary to generate a multilevel resistprofile. These approaches, however, have limited resolution since silverhalide-based photographic emulsion is used and the grayscale mask is ahalftone mask, that is not a true gray scale mask.

[0006] The fabrication of microoptical elements such as refractivemicrolens arrays, diffractive optical elements, prism couples, andthree-dimensional microstructures in general can be realized with theexisting micro-fabrication methods normally used for the production ofmicroelectronics. The well-established microfabrication technologiesinclude photolithographic process and reactive ion etching.Photolithographic processes are employed to print a mask pattern in aphotomask onto photoresist film, which is typically coated on a siliconwafer or a glass wafer. Commercially available photolithographicprinters for microfabrication include contact and proximity printers, 1×projection printer, 1× steppers and 5× as well as 10× reductionsteppers. Reactive ion etching is employed to transfer and/or replicatepatterns in photoresist into the underlying substrate material.Commercially available systems include plasma etchers, inductive coupledplasma (ICP) and chemically assisted ion beam etchers (CAIBE).

[0007] For the fabrication of integrated circuits (IC) inmicroelectronics industry, a set of binary masks is used in thephotolithographic process. The binary masks typically have IC patternsdefined in a chrome film which is coated on a silicate glass plate,typically a fused silica glass plate. However, for the fabrication ofmicrooptical elements, a gray scale photomask is needed to define thethree dimensional microstructures.

[0008] A gray scale photomask carries patterns with areas of differenttransmittance. When the pattern is printed on photoresist, areas ofdifferent transmittance in the gray scale mask create areas of differentthickness in photoresist after development. Therefore, a gray scalepattern in a gray scale photomask can be used to create predetermined 3Dmicrostructures in photoresist film, which are then transferred and/orreplicated into the underlying substrate material in a reactive ionetcher.

[0009] Instead of using a gray scale photomask a varied exposure in aphotoresist can also be generated by directly exposing the photoresistwith an e-beam writer or a laser beam writer. The developed 3D resiststructure can then be transferred into underlying substrate material toproduce microoptical elements. However, in this case no mask is created.Each element must be written one at a time, with no benefit fromeconomies of scale. Namely, it is not cost effective for makingmicrooptical elements in production quantities using this direct writemethod.

[0010] There have been several methods of making gray scale photomasksin the past, but each of them have a major shortcoming as describedbelow.

[0011] U.S. Pat. Nos. 5,480,764 and 5,482,800 of Gal et al and anarticle by W. W. Anderson et al “Fabrication of Micro-optical Devices”conference on Binary Optics 1993, pp. 255-269, teach half tone grayscale masks. According to this technique, the mask is created byconstructing a plurality of precisely located and sized openings, thefrequency and size of these openings produce the desired gray scaleeffect provided the mask pattern is blurred in the photolithographicprocess to print on photoresist. The smallest features of this mask arebinary, either open or closed, i.e. on or off. A group of a large numberof on and off spots is needed to create a gray scale resolution element.The gray scale resolution element appears for example as 80%transmittance or as 20% transmittance depending on the ratio of thenumber of the on-spots and off spots. Therefore, the resolution of ahalf-tone gray scale pattern is much reduced from that of a binarypattern in a chrome mask.

[0012] Photographic emulsion has been used to provide gray scale masks.A gray area consists of a number of silver grains and openings. Thesilver grains are totally opaque and the openings are totallytransparent. Therefore, the photographic gray scale mask is also ahalf-tone mask. The gray scale resolution element of a photographic filmis in general larger in size than that of the halftone gray scale chromemask. This is because the silver grains in a developed photographicemulsion film are in general larger than an opening or a chrome spotthat can be made in a chrome mask.

[0013] One improvement in the production of gray scale masks for use infabricating microoptical elements has been realized with the provisionof a gray scale mask wherein different thicknesses of a light absorbingmaterial, such as Inconel are coated on a glass plate to form the grayscale mask (see U.S. Pat. No. 6,071,652 of Feldman, et al, Jun. 6,2000). This gray scale mask could have the high resolution required forfabricating microoptical elements. However, one disadvantage of thistechnique is the cost of the mask generation, wherein multiple directwrite steps on photoresist are required to provide the lift off processof the light absorbing material for each discrete thickness desired. Thetight thickness control necessary in the material evaporation stepsmakes this technique economically undesirable.

[0014] The gray scale photomasks described above cannot be utilized forthe fabrication of high quality micro-optical elements in productionquantities, because of their failure to satisfy either one or both ofthe following requirements: 1.) a sub-micrometer gray scale resolutionelement, 2.) acceptable cost in the mask generation.

[0015] High Energy Beam Sensitive (HEBS) glasses were described in U.S.Pat. Nos. 4,567,104, 4,670,366, 4,894,303, 5,078,771, and 5,285,517 (Wupatents herein after) by Che-Kuang Wu who is also known as Chuck Wu, thelatter name is used in some of his publications in technical journals.The size of a gray scale resolution element in an HEBS-glass gray scalemask is limited only by the e-beam darkened spot size recorded in thehigh energy beam sensitive glass plate. It is typically 0.1 micrometerto 0.4 micrometer depending on the acceleration voltage of the electronbeam and on the electron dosage. It is obvious that an HEBS-glass grayscale mask satisfies the high resolution requirement. However, theHEBS-glass plates of Wu patents did not satisfy the second requirementlisted in the paragraph immediately above. The reason for not being ableto satisfy the requirement of acceptable cost in the mask generation iselaborated below:

[0016] 1. To render HEBS-glass of Wu patents opaque, the required e-beamdosage is typically about 1,000 microcoulomb/cm². This should becompared with the sensitivity of the electron beam resist used in ICindustry to fabricate the binary chrome photomask. The required e-beamdosage for the electron beam resist employed by the mask shops in ICindustry ranges from 0.1 to 1 microcoulomb/cm². HEBS-glass of the Wupatents has an e-beam sensitivity that is a factor of 1,000 to 10,000times less than that of electron beam resist used in IC industry. 1,000to 10,000 times less in sensitivity is herein after referred to as thesensitivity factor. Relative to photographic emulsion, HEBS-glass isalso very insensitive. This is expected because there is not adevelopment step for the HEBS-glass to enhance the contrast of thee-beam exposure induced optical density. The amplification in theoptical density of a photographic film by a chemical development step isa factor of 10⁷. Namely there is a contrast enhancement of ten milliontimes from the latent image to the chemically developed image in aphotographic emulsion film.

[0017] E-beam writers are very expensive, the write time of a mask hasto be in the order of minute or hours, not days, otherwise there wouldbe little economic value. E-beam exposure systems that are commerciallyavailable include flood electron beam exposure systems, raster scane-beam pattern generators, variable shape beam vector scan e-beampattern generators and Gaussian spot vector scan e-beam patterngenerators. The price of the systems together with the write time forgenerating a mask determines the cost of making an HEBS-glass mask.

[0018] Flood e-beam exposure system, for example, EVC Electron Curesystems manufactured by Electron Vision Corp. are priced at about$250,000 each. Raster scan e-beam pattern generators, for example, MEBESsystems manufactured by ETEC are priced at more than ten million dollarseach. Variable shaped beam vector scan e-beam pattern generators, forexample ZBA 23H manufactured by Leica Microsystems, Inc. are priced atabout $6 million each. Gaussian spot vector scan e-beam patterngenerators such as Vector Beam manufactured by Leica Microsystems, Inc.are priced at more than $6 million each.

[0019] Flood e-beam exposure system is ideally suited for HEBS-glasscomposition research for comparing the e-beam sensitivity among manyglass compositions via uniformly exposing the entire area of manyHEBS-glass plate samples in one exposure to an identical electrondosage.

[0020] However, flood e-beam exposure system, which is much lessexpensive, is not an option for making a photomask for the followingreason. EVC Electron Cure systems has no capability of delivering ane-beam dosage having a predetermined functional variation in x and ycoordinates. An Electron Cure 30 flood e-beam exposure system providesan 8 inch diameter beam to uniformly expose an area of up to 8 inchdiameter. HEBS-glass of Wu patents are typically darkened to 1 unit ofoptical density value under the flood gun exposure for about 10 minutes.If one aperture down the beam size of the EVC Electron Cure system to0.1 micron spot to write a gray scale pattern in HEBS-glass, it wouldtake 10¹⁰ minutes or in other words, 6.9 million days, to expose apattern size of 1 cm×1 cm area. Moreover, there exists no fixture in EVCElectron Cure system for precision movements of an aperture in the X-Yplane to create a mask pattern. U.S. Pat. No. 5,468,595 of William R.Livesay teaches a method of controlling the solubility of photoresistlayer in the depth dimension, i.e. Z-axis, through flood exposure onphotoresist a uniform electron dosage in X-Y plane. This is accomplishedby the electron beam having a controlled acceleration voltage. Thismethod is not applicable and is undesirable for making a HEBS-glass grayscale photomask for the following reason. The gray scale patterns in aphotomask have to be in one plane (X-Y plane) without variation in thethickness dimension so that all patterns in the mask can be in focussimultaneously during the photolithographic printing process.

[0021] During the months of March and April of 1987, Motorola, one ofthe world's largest manufacturers of integrated circuits (i.e. ICchips), evaluated the HEBS-glass plates of Wu patents for use as abinary mask for IC photomask applications and concluded that HEBS-glassplates of Wu patents require too much e-beam write time creating a highcost of mask generation. This deterred their use of HEBS-glassphotomask. Motorola's evaluation report of Jun. 25, 1987 stated“Numerous hours of e-beam write time are required to produce onephotomask. This condition is totally unacceptable on Motorola's presentand future mask making plans.” This began the search for newapplications for HEBS-glass of Wu patents. The possibility of making agray scale photomask using HEBS-glass of Wu patents was pursued. In1988, gray scale test patterns were written with a MEBES e-beam writerusing the number of retraces as a variable parameter to vary theelectron dosage. CMI purchased an EVC Electron Cure 30 flood e-beamexposure system and a Hitachi Spectrophotometer Model u2000 for thepurpose of HEBS-glass composition research, in search of a much improvede-beam sensitivity. Glass batches of a large number of different glasscompositions were melted in alumina crucibles. Ground and polished glassplates from the glass melts were ion-exchanged in acidic aqueoussolution containing silver nitrate to produce HEBS-glass plates. Groupsof 5 to 10 HEBS-glass plates, typically in sizes up to about 0.5″×1″each were exposed together in the 8″ diameter electron beam of theElectron Cure 30 with an electron dosage of 400 microcoulomb/cm2.Spectral absorption curves of the darkened HEBS-glass plates weremeasured with Hitachi Spectrophotometer Model u2000 in order to comparethe e-beam sensitivity among the compositions of glass melts for furtheriterations of glass melts in the optimization of the HEBS-glasscompositions.

[0022] 2. The industrial standard mask making tool is a MEBES e-beamwriter. In the IC industry, only binary photomasks are required, and allcommercial photomask shops in the U.S. use MEBES systems to write thebinary photomasks for the fabrication of IC chips. The MEBES system is araster scan system. An electron beam is raster scanned in a serialmanner to each and every address of for example 10¹⁰ addresses in a 1cm×1 cm area when a 0.1 micron addressing grid size is chosen. Patternsare generated by blanking or un-blanking the beam at each address. Theraster scan is done at a fixed constant rate for each MEBES system; forexample at 160 MHz rate, the e-beam dwell time at each address is 6.25nanosecond. To produce a gray scale pattern in a HEBS-glass plate, it isnecessary that the electron dosage at each address is predetermined bythe design of the gray scale pattern. Since the MEBES system have aconstant dwell time of 6.25 nanoseconds for each of all addressing gridpoints and the e-beam dwell time cannot be varied from one address tothe next address, a gray scale pattern in a HEBS-glass plate can only begenerated with multiple retraces. This exposure scheme is impracticalfor making HEBS-glass gray scale photomasks.

[0023] A gray scale pattern of continuously varying optical density on aHEBS-glass plate requires a large number of gray scale levels. Grayscale patterns having more than 1,000 gray levels can be produced in aHEBS-glass plate when a practical exposure scheme becomes available.Using a raster scan e-beam writer, the write time of writing one grayscale level in a HEBS-glass plate would be the same as that of writing abinary chrome mask if the e-beam sensitivity of a HEBS-glass plate isidentical to that of an electron beam resist. However, the write time ofgenerating a gray scale photomask in a HEBS-glass plate is the writetime of a binary chrome mask multiplied by the sensitivity factor of1,000 to 10,000 and then multiplied by the number of gray scale levels.For a HEBS-glass gray scale photomask having 1,000 gray scale levels,1,000 retraces is required and the write time would be at least1,000,000 times that of writing a binary chrome mask. In other words,the throughput of making a HEBS-glass mask with 1,000 gray levels is1,000,000 times lower relative to making a binary chrome mask. The writetime of a binary chrome mask is typically more than 1 hour at a cost ofabout $1,000 per hour of e-beam write time. The cost of e-beam writetime, e.g. 1,000,000 hours to write 1 plate clearly prohibits the use ofa HEBS-glass plate of Wu patents to make a gray scale photomask.

[0024] Besides the prohibitive cost, the technical feasibility of makinga HEBS-glass gray scale mask is doubtful due to the properties ofHEBS-glass described immediately below:

[0025] 1. The heat effect of HEBS-glass is described in the section“Heat Effect of the Write Beam” in this application, and is alsodescribed in an article co-authored by Wu and E. B. Kley et al “Adaptingexisting e-beam writers to write HEBS-glass gray scale masks” inProceedings of SPIE Vol. 3633 (January 1999). The heat effect increasesthe sensitivity of HEBS-glass, but the heat effect is a strong functionof exposure beam sizes and shapes. As a consequence, for a constante-beam exposure dosage, the e-beam induced optical density in HEBS-glassis not a constant value and is a function of exposure beam size andshape. This property of HEBS-glass restricts the utilization of theadvantages of the exposure scheme inherent in a variable shape beamsystem for increasing the throughput of writing a mask pattern.

[0026] 2. The e-beam darkening mechanism of HEBS-glass includes anintermittence effect in addition to the heat effect. The e-beamdarkening mechanism is not known with certainty and is postulated asfollows. In the presence of a high energy electron beam (e-beam), someof Cl− ion and Ag+ ion in the silver halide alkalihalide complex crystalor complex microphases in the integral ion exchanged surface glass layerof a HEBS-glass plate react with energetic electrons to produce Cl atomand Ag atom. Cl atom and Ag atom are not stable species and a reversereaction takes place simultaneously. A third reaction process alsooccurs simultaneously wherein portions of Cl atom and Ag atom becomestable species of Cl₂ and Silver specks Ag_(n) with the help of latticevibrations as described in the section “Heat Effect of the Write beam”in the above application.

[0027] If the e-beam exposure is done with intermittence such as anexposure with multiple retraces, the e-beam induced optical density in aHEBS-glass plate resulting from a constant total exposure dosage ofmultiple retraces is not a constant value, but is a function of theintermittence time duration. This is because the intermittence timeduration contributes additional time duration for the formation of thestable species of Cl₂ and silver specks and retards the reverse reactionof Cl atom and Ag atom back to Cl− ion and Ag+ ion, due to the reducedconcentration of Cl atom and Ag atom. Due to the intermittence effect,exposure schemes with multiple retraces is complicated by the additionalvariable parameter, the intermittence time duration. The intermittenceeffect is described in the article co-authored by Wu and E. B. Kley etal “Adapting Existing E-beam Writer to Write HEBS-glass Gray ScaleMasks” in Proceedings of SPIE vol. 3633, January 1999.

[0028] In the IC industry, direct write on photoresist to generatebinary IC patterns is benefited from the choice of a variable shapedbeam system to increase the throughput of pattern generation. However,for the fabrication of HEBS-glass photomasks, an exposure schemeutilizing a variable shaped beam does not produce a constant value ofe-beam induced optical density in HEBS-glass for a constant e-beamexposure dosage, particularly when a high e-beam current density isused. Therefore, the fabrication of HEBS-glass gray scale photomasksusing an exposure scheme with a variable shaped beam requires multipleretraces and a low current density, and thus the throughput of writingthe HEBS-glass mask is further reduced.

[0029] MEBES e-beam writers, the only e-beam writers commerciallyavailable for mask writing service, do not provide a practical exposurescheme for making HEBS-glass gray scale masks. A raster scan e-beamsystem can write at a very high data rate, which is ideally suited forwriting a binary mask. For a gray scale mask requiring many gray scalelevels such as 1,000 gray scale levels, 1,000 retraces is needed towrite 1 mask using a raster scan e-beam writer. A vector scan e-beamwriter with a capability of changing dwell time at each address on thefly may not require multiple retraces to write a HEBS-glass gray scalemask. Therefore, C. Wu set forth to look for e-beam writing tools thatare designed for R&D purposes in the universities. CMI (CanyonMaterials, Inc.) product information No. 94-88 “HEBS-glass PhotomaskBlanks” was prepared by C. Wu in December 1994, for the purpose ofencouraging researchers and e-beam operators in the university to testwrite HEBS-glass plates.

[0030] On Apr. 13, 1995, C. Wu visited Mr. Robert Stein, who was ane-beam operator at UCSD (University of California, San Diego), andshowed Mr. Stein e-beam written plates of HEBS-glass. The HEBS-glassplates were written with a MEBES e-beam writer. Test patterns withvariation of optical density were written with different numbers ofretraces.

[0031] After explaining to Mr. Stein the e-beam direct write phenomenon,Mr. Stein agreed to test write on a HEBS-glass plate using an EBMF 10.5e-beam writer.

[0032] By May 24, 1995, Mr. Stein finished writing a HEBS-glass platewith EBMF 10.5 e-beam writer using various beam currents at 30 kv and at20 kv. An addressing grid size of 0.1 micron was employed. Sixteen graylevels were generated with each setting of beam current and kvcombinations using 16 clock rates.

[0033] C. Wu had Mr. Walter Daschner examine the gray scale pattern inthe HEBS-glass plate written by Mr. Stein, and met with Mr. Daschner, agraduate student under Professor S. H. Lee, on Jun. 12, 18995 at 10 AMat UCSD for the first time. During the meeting C. Wu presented to Mr.Daschner a copy of CMI Product Information No. 94-88 “HEBS-glassPhotomask Blanks,” one plate of a HEBS-glass photomask blank, and a copyof U.S. Pat. No. 5,078,771 “Method of Making High Energy Beam SensitiveGlass.”

[0034] This meeting led to joint research and the following publicationsthat C. Wu co-authored with others:

[0035] A. “General aspheric refractive micro-optics fabricated byoptical lithography using a high energy beam sensitive glass gray-levelmask” by Walter Daschner, Pin Long, Robert Stein, Chuck Wu, and S. H.Lee, in J. Vac. Sci. Technol. B 14(6), November/December 1996.

[0036] B. “Cost-effective mass fabrication of multilevel diffractiveoptical elements by use of a single optical exposure with a gray-scalemask on high energy beam-sensitive glass” by Walter Daschner, Pin Long,Robert Stein, Chuck Wu, and S. H. Lee, in Applied Optics, Vol. 36, No.20, Jul. 10, 1997.

[0037] CMI Product Information No. 94-88 was cited as reference No. 11,and as reference No. 7 in the above listed publication A ofNovember/December 1996 and B of Jul. 10, 1997, respectively.

[0038] Based on the results of written HEBS-glass plates by Mr. Stein,C. Wu, being the Chairman of Canyon Materials, Inc., caused CanyonMaterials, Inc. to purchase a EBMF 10.5 e-beam writer from LeicaMicrosystems, Inc. for the purpose of developing an e-beam exposurescheme and optimizing e-beam write parameters for making gray scalephotomasks using HEBS-glass plates, and for commercializing HEBS-glassgray scale photomasks. Before the EBMF 10.5 e-beam writer becameavailable in-house, CMI purchased write time of EBMF 10.5 e-beam writerfrom UCSD for the above stated purposes.

[0039] C. Wu also conducted other efforts to write HEBS-glass platesusing other e-beam writers and efforts to write LDW glass plates usinglaser beam pattern generators through joint efforts with other researchinstitutions and universities in the US as well as abroad. These otherefforts result in, for example, the following 3 publications that C. Wuco-authored:

[0040] C. “Adapting existing e-beam writers to write HEBS-glass grayscale masks” by E.-Bernhard Kley, Matthias Cumme, Lars-Christian Wittig,and Chuck Wu, in Proceedings of SPIE Vol 3633, Janarnay 1999.

[0041] D. “Applications of gray scale LDW-glass masks for fabrication ofhigh-efficiency DOEs” by V. P. Korolkov, A. I. Malyshev, V. G. Nikitin,A. G. Poleshchuck, A.

[0042] A. Kharissov, V. V. Cherkashin, and C. Wu, in Proceedings of SPIEVol. 3633, January 1999.

[0043] E. “Fabrication of gray scale masks and diffractive opticalelements with LDW-glass” by Victor Korolkov, Anatoly Malyshev, AlexanderPoleshchuk, Vadim Cherkashin, Hans Tiziani, Christof Prub, ThomasSchoder, Johann Westhauser, and C. Wu, in Proceedings of SPIE Vol. 4440,July 2001.

[0044] It is not at all obvious and is a total surprise that the highthroughput of e-beam writing HEBS-glass gray scale photomasks such asmask No. 81, 82, 83, and 84 were produced. The surprise is that the1,000,000 times too low throughput of making a HEBS-glass mask has beenovercome by the combined efforts of the glass compositions of thisapplication and an e-beam exposure scheme optimized for the propertiesof the HEBS-glass of this application. Mask No. 81, 82, 83, and 84 werewritten with an EBMF 10.5 e-beam writer using the write parametersnecessary for producing the combined effects. These write parameters ofthe exposure scheme are described in the section “Description of theInvention” of this application. EBMF 10.5 is a Gaussian spot vector scane-beam writer manufactured by Leica Microsystems, Inc. This e-beamwriter is a research tool and is not available in commercial mask shopsfor IC photomask fabrication. The write parameters necessary forproducing the combined effects of this application has never been andcan never be applied to expose electron beam resists for which thee-beam writers were designed. This is because the e-beam power thatgenerates the heat effect for the enhanced e-beam sensitivity inHEBS-glass, would burn or decompose the electron beam resist. The e-beampower is the input-power density defined as (beam current)×(beamacceleration voltage)/(beam spot size) in the section “Heat Effect ofthe Write E-Beam” of this application.

[0045] HEBS-glass gray scale mask No. 81 having 1,000 gray scale levelswas written in 1 hour, 14 minutes, and 26 seconds. This gray scale maskis fabricated for making 50 copies of 100 micron×6 mm prism couples ineach contact print on wafer. Each prism couple has a right angletriangular cross section of 2 micron height and is 6 mm long. The grayscale pattern for each prism has 1,000 gray levels. There is no otherproduct, apparatus, or method that could produce such a gray scale maskat the cost of a HEBS-glass mask.

[0046] HEBS-glass gray scale mask No. 82 having 100 gray scale levelswas written in 57 minutes 35 seconds. This gray scale mask is fabricatedfor making blazed gratings having a 20 micron pitch in fused silicaglass wafer using a contact printing process. The 20 micron pitchgrating is 10 mm long, and has 500 periods. The grating is 1 cm×1 cm insize.

[0047] HEBS-glass gray scale mask No. 83 having 23,040 lens patterns,each of 100 gray scale levels was written in 13 hours 12 minutes and 1second. A large portion of the e-beam write time is consumed by dataloading since circular patterns with a large number of gray levelsrequire a very large data file. This gray scale mask is fabricated formaking 1×40 arrays of refractive microlenses for fiber opticalinterconnect. Each lenslet has 200 micron diameter and 100 gray levels.There are 576 dies in the mask pattern, each die being a 1×40 lensarray.

[0048] HEBS-glass gray scale mask No. 84 having 62,500 lens patterns,each of 57 gray scale levels, was written in 3 hours and 36 minutes.This gray scale mask is fabricated for making a 250×250 array ofrefractive microlenses. Each lenslet is a 40 micron×40 micron squarelens having 57 gray levels in the HEBS-glass mask. The array has 100%fill factor for use in detector enhancement. To create an array ofsquare lenses (100% fill factor), a circular lens whose diameter is thediagonal of the square, i.e. 56.56 micron in this case, with theappropriate number of gray levels is created, trimmed into a square, andstepped and repeated to create the lens array. Each gray level of themask is a layer in the data file.

[0049] Based on the postulated model of e-beam darkening mechanism inHEBS-glass described herein, high throughput of e-beam patterningHEBS-glass gray scale photomasks is made possible as follows:

[0050] 1. In the section of this application “Heat Effect of Write Beam”the e-beam darkening mechanism is elaborated. The formation of a silverspeck consisting of 2, 3, or more atoms requires the deformation ofsilver halide lattice to silver lattice. Cycles of lattice vibration ofsufficient amplitudes are necessary to cause the formation of the silverspecks. Since larger amplitudes of lattice vibrational modes exist athigher temperatures, silver specks are formed more quickly at a highertemperature. HEBS-glass compositions No. 1 to No. 20 represent theHEBS-glass compositions of this invention having produced silver halidealkalihalide complex crystals, in the integral ion exchanged surfaceglass layer of the HEBS-glass plates, that are optimized to maximize thee-beam sensitivity enhancement of the heat effect. The relativeconcentrations as well as the total concentration of alkali oxides, i.e.Li₂O, Na₂O and K₂O are among the more important parameters of the baseglass composition that determine the heat enhanced e-beam sensitivity ofthe silver halide alkalihalide complex crystals. Other variableparameters of the HEBS-glass compositions of this application arerepresented in Exhibit A of the application.

[0051] 2. The acceleration voltage of an electron beam among all thecommercially available e-beam writers ranges from 1 kV to 100 kV. Inother words, the kinetic energy of electrons in the e-beam writer rangesfrom 1 keV to 100 keV. When a high energy electron enters into any solidmaterial, i.e. HEBS-glass in this application, it creates secondaryelectrons and third generation electrons due to electron-electroncollision. For example, a 100 keV electron penetrating HEBS-glass couldin principle produce up to 100,000 energetic electrons, each having akinetic energy of 1 ev on the average. The secondary and thirdgeneration electrons are the energetic electrons that cause the chemicalreaction of Cl− ion and Ag+ ion to form Cl atoms and Ag atom. A higherkV electron beam creates a larger number of energetic electrons.However, the bulk of the energetic electrons exists deeper into thethickness dimension from the HEBS-glass surface as the accelerationvoltage of e-beam increases. By adjusting the thickness dimension, i.e.x₁ and x₂ (see FIG. 1 of the Application) of the ion exchanged surfaceglass layer of a HEBS-glass plate, the bulk of the energetic electronscan be captured within the e-beam sensitized glass layer. However, toproduce a gray scale photomask with a high resolution capability in aphotolithographic process, a smaller value of (x₂−x₁), such as less than3 micron is necessary. Therefore, for each of the glass compositions No.1 to No. 20 of Exhibit A of the Application, x₁ and x₂ values wereoptimized for a maximum sensitivity to 20 kV electron beam to produceHEBS-glass plate No. 1 to No. 20. Although HEBS-glass plate No. 1 to No.20 were optimized for the penetration depth of a 20 kV electron beam, a30 kV electron beam in general produces a higher OD (Optical Density)value with the same electron dosage. This is due in part to a strongerheat effect described above.

[0052] High throughput of e-beam patterning HEBS glass gray scale masksis made possible by maximizing the heat enhanced e-beam sensitivity ofthe HEBS-glass compositions of this application and writing gray scalepatterns in HEBS-glass plates with an exposure scheme that produced thecombined effects described herein. The exposure scheme employed for thehigh throughput writing of HEBS-glass plates is not available from thee-beam writers such as MEBES that are typically utilized by IC photomaskshops.

[0053] U.S. Patents Nos. 4,567,104; 4,670,366; 4,894,303; and 078,771and 5,285,517, all of inventions of Che-Kuang Wu, described High EnergyBeam Sensitive glass (HEBS-glass) articles exhibiting insensitivityand/or inertness to actinic radiation, the HEBS-glass articles which aredarkened and/or colored within a thin surface layer of about 0.1 - 3micron upon exposure to a high energy beam, electron beam, and ion beamsin particular, without a subsequent development step, and which need nofixing to stabilize the colored image, since both the recorded image andthe glass article are insensitive to radiation in the spectral range ofuv and longer wavelengths. These patents are concerned with Ag+ion-exchanged glass articles having base glass within alkali metalsilicate composition fields containing at least one of the oxides oftransition metals which have one to four d-electrons in an atomic state.The base glass composition can be varied widely, spontaneous reductionupon ion-exchange reaction as well as photo-reduction of Ag+ ions areinhibited and/or eliminated due to the presence of said transition metaloxides in the glass article. The HEBS-glass is suitable for use asrecording and archival storage medium and as phototools. The recordedimages and/or masking patterns are up-dateable, can be any single colorseen in the visible spectrum, and is erasable by heat at temperaturesabove 200° C. Heat erasure mode of recording the high energy beamdarkened glass article using a high intensity light beam, focused laserbeam in particular, was also described.

[0054] Diffractive optics technology is maturing see for example, thepublication by C. W. Chen and J. S. Anderson, “Imaging by diffraction:grating design and hardware results,” in Diffractive and MiniaturizedOptics, S. H. Lee, ed., Vol. CR49 of SPIE Critical Reviews Series(Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash.,1993) pp. 77-97. Diffractive Optical Elements (DOE's) of various designshave been found useful for improving the design and performance ofoptical systems. Instead of using the binary method, such as describedin “Binary Optics Technology: The Theory and Design of Multi-level PhaseDiffractive Optical Elements” by G. J. Swanson of MIT, documented in MITTech. Rep. 854. (MIT, Cambridge, Mass., 1989), a cost-effective way offabricating large numbers of DOE's in the shortest possible turnaroundtime has become increasingly important. Gray scale mask fabricationmethods offer these features by drastically reducing the amount ofprocessing steps involved to generate a multilevel and monolithic DOE.Currently multiplexing schemes exist to fabricate a quasi-gray-scalemask by changing the number of area openings in a binary mask (similarto the halftone method) or by photographic emulsions. These approacheswere described by Y. Opplinger et al. in Microelectron Eng. 23, 449-454(1994) and by H. Anderson et al. in Appln. Opt. 29, 4259-4267 (1990).Other methods of fabrication of gray-scale masks involve the cumberstonetask of multiple binary exposures and following evaporation steps suchas described by W. Daschner, et al. in J. Vac. Sci. Technol. B 13,2729-2731 (1995). The High Energy Beam Sensitive (HEBS) glass of thepresent invention offers the advantage of a one-step fabrication of truegray-scale masks.

[0055] The continuing development of exceedingly small or so-calledmicro-devices such as micro-optic elements and micro-machines is ofgreat importance to optoelectronic interconnection technologies and thedevelopment of communications and control systems. Diffractive opticalelements such as spherical, cylindrical, Fresnel lenses, aspherics andother micro-devices having rather precise three dimensional profiles orcontours present certain problems with respect to volume production ofthese elements of an acceptable quality, in particular. The fabricationof large arrays of such elements covering large areas is very costlywith regard to known methods of production.

[0056] One technique for mass production of diffractive optical elementsinvolves fabricating a master element which itself is made by etchingprocesses similar to those used in the fabrication of micro-electroniccircuits and similar devices wherein a multi-masking process usingbinary masks is conducted. The fabrication of a master or individualelements using a multi-binary mask method can result in significantdimensional errors in the master and the fabricated element due toresidual alignment errors between consecutive masking steps. Althoughdiamond turning, for example, can be employed in producing a masterelement, the multi-binary mask technique is limited with symmetricelements, for example. Still further, diffractive optical elements canbe produced by injection molding, embossing or casting. However, thematerials used in these techniques have limited optical andenvironmental properties, and are, for example, operable to betransmissive to radiation only in the spectral range visible to thehuman eye.

[0057] Some of the disadvantages of prior systems including thosementioned above have been overcome with the development of so-calledgray scale masks which avoid multiple processing steps by providing asingle mask which contains all of the information necessary forgenerating multi-phase levels, i.e., the three-dimensional contoursrequired in a diffractive optical element and the like. Photographicemulsions have been used to provide gray scale masks which can begenerated using a laser writer or an optical imaging system, forexample. However, the high resolution required of diffractive opticalelements and other micro-elements is limited with this technique due tothe limited resolution of the laser writer and the graininess of theimage on the emulsion based mask. Moreover, photographic emulsions arenot particularly durable and do not allow cleaning of the mask withwater or mechanical scrubbing.

[0058] Other gray scale masking techniques, including the so-called halftone binary mask, are also limited due to the small holes in the maskwhich will also diffract light passing through the mask, furtherlimiting the resolution of the desired diffractive optical element, forexample.

[0059] One improvement in the production of gray scale masks for use infabricating diffractive optical elements and other micro-elements hasbeen realized with the provision of a gray scale mask wherein differentthicknesses of a light-absorbing material, such as Inconel, coated on aglass plate mask element, for example, can provide for the fabricationof a gray level mask with high resolution and compatibility withsubstantially all wavelength ranges used in optical lithography.However, one disadvantage of this technique is the cost of the maskgeneration method wherein multiple direct write steps are required toprovide the lift off process of the light-absorbing material for eachdiscrete thickness desired. The tight thickness control necessary in thematerial evaporation step makes this technique somewhat economicallyinfeasible for many applications.

[0060] The use of a gray scale mask fabrication method for producinglarge quantities or large arrays of diffractive optical elements andsimilar micro-elements requiring high resolution of three dimensionalcontours has several advantages. Gray scale masks require only a singleexposure of a photoresist when fabricating the elements on a substrateusing an etching process. Gray scale masks thus avoid the alignmenterrors resulting from processes requiring the use of multiple binarymasks. Moreover, if a suitable gray scale mask material is provided,thermal expansion and contraction of the mask can also be avoided.

[0061] Accordingly, there has been a continuing need to develop animproved fabrication method for relatively large quantities of and largearrays of micro-elements, such as diffractive optical elements or otherelements covering large areas, such as computer generated holograms. Itis to these ends that the present invention has been developed.

SUMMARY OF THE INVENTION

[0062] Since there is no graininess, HEBS-glass is capable of resolutionto molecular dimensions. HEBS-glass turns dark instantaneously uponexposure to an electron beam, the more electron dosage the more itdarkens. Therefore, HEBS-glass is ideally suited for making gray levelmasks. HEBS-glass gray level masks can be written with an e-beam writerusing a 0.1 μm addressing grid size. Every 0.1 μm spot in the 5″×5″HEBS-glass plate acquires a predetermined transmittance value rangingfrom 100 percent down to less than 0.1 percent upon e-beam patterningwith a predetermined dosage for each address. A gray level mask made ofHEBS-glass does not relay on a halftone method. Therefore, it is a truegray level mask.

[0063] It is the objective of the present invention to design HEBS-glasscompositions so that the HEBS-glass gray level mask of the presentinvention facilitates new designs and low cost manufacturing processesfor high-performance diffractive optics; asymmetric, irregularly shapedmicrolens arrays; and general three dimensional surfaces.

[0064] Application of the HEBS-glass of the present invention includemicro-optical devices, microelectrical devices,micro-opto-electromechanical devices, integrated optical devices,two-dimensional fanout grating, optical interconnect, fiber pigtailing,diffractive optical elements, refractive microlens arrays, microprismarrays, micromirror arrays and Bragg grating.

[0065] The essential properties of a HEBS-glass gray level mask which isnecessary for the fabrication of general three dimensionalmicrostructures are:

[0066] 1. A mask pattern or image is grainiless even when observed underoptical microscope at 1 000× or at higher magnifications.

[0067] 2. The HEBS-glass is insensitive and/or inert to photons in thespectral ranges employed in photolithographic processes, and is alsoinsensitive and/or inert to visible spectral range of light so that aHEBS-glass mask blank and a HEBS-glass mask are permanently stable underroom lighting conditions.

[0068] 3. The HEBS-glass is sufficiently sensitive to electron beamexposures, so that the cost of making a mask using an e-beam writer isaffordable for many applications.

[0069] 4. The e-beam induced optical density is a unique function of,and is a very reproducible function of electron dosages for one or morecombinations of the parameters of an e-beam writer. The parameters ofe-beam writers include beam acceleration voltage, beam current, beamspot size and addressing grid size.

[0070] The essential properties No. 1 and No. 2 are properties ofHEBS-glasses described in the US patents listed above. However,HEBS-glass compositions having a better e-beam sensitivity is in generalmore sensitive to photon energy as well.

[0071] It is the objective of the present invention to optimizeHEBS-glass composition so that the HEBS-glass of the present inventionis sufficiently sensitive to electron beam and that the cost of writinga gray level mask is affordable for many applications, and yetHEBS-glass of the present invention is totally inert to actinicradiation of 436 nm and longer wavelengths and has no sensitivity toactinic radiation at 365 nm for practical purposes, eg. no significantdarkening for 1,000,000 exposures in I-line steppers.

[0072] It has been determined that with a given value of e-beam exposuredosage the e-beam induced optical density in HEBS-glass is a function ofbeam acceleration voltage, of beam spot size, of beam current and ofaddressing grid size. Therefore, it is another objective of the presentinvention to design e-beam write schemes such as that the essentialproperties No. 3 and No. 4 of a HEBS-glass gray level mask are bothfulfilled.

[0073] The present invention is directed to a gray scale mask comprisinga transparent High Energy Beam Sensitive-glass (HEBS-glass) having atleast one gray scale zone with a plurality of gray scale levels, eachgray scale level having a different optical density, the High EnergyBeam Sensitive-glass in bodies of 0.090 inch cross section will exhibitthe following properties:

[0074] (a) transmittance of more than 88% at 436 nm; and

[0075] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm², said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂.

[0076] In one embodiment, at least one gray scale zone has a continuousgray scale comprising a plurality of grade scale levels.

[0077] The present invention is also directed to a method of making agray scale mask comprising writing on a plurality of areas on at least aportion of a High Energy Beam Sensitive-glass (HEBS-glass) with anelectron beam having an acceleration voltage of 20 to 30 kilovolts, abeam current of 25 to 175 nanoamps, and addressing a grid size of 0.1 to0.4 micron; the writing carried out at an electron dosage that falls onthe net optical density vs. electron dosage sensitivity curve of theHigh Energy Beam Sensitive-glass, the initial slope of the sensitivitycurve being from 2.454 to 12.507 per electron dosage unit ofmilli-coulombs/cm²; the exposure duration of the writing on each areaare different than the exposure duration of the immediate adjacentareas; the High Energy Beam Sensitive-glass in bodies of 0.090 inchcross section will exhibit the following properties:

[0078] (a) a transmittance of more than 88% at 436 nm; and

[0079] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm²; said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂.

[0080] The present invention is also directed to a method of making athree dimensional microstructure with three dimensional surfaces in aphotoresist comprising exposing a photoresist to a gray scale pattern ina gray scale mask using an optical lithography tool and developing theexposed photoresist to form three dimensional microstructures in thephotoresist;

[0081] the gray scale mask comprising a transparent High Energy BeamSensitive-glass (HEBS-glass) having at least one gray scale zone with aplurality of gray scale levels, each gray scale level having a differentoptical density, the High Energy Beam Sensitive-glass in bodies of 0.090inch cross section will exhibit the following properties:

[0082] (a) a transmittance of more than 88% at 436 nm; and

[0083] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm²; said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0084] The present invention is also directed to an analog photoresistwith a three dimensional microstructure produced by exposing aphotoresist to a gray scale pattern in a gray scale mask using anoptical lithography tool and developing the exposed photoresist to formthe three dimensional microstructure in the photoresist; the gray scalemask comprising:

[0085] A gray scale mask comprising a transparent High Energy BeamSensitive-glass (HEBS-glass) having at least one gray scale zone with aplurality of gray scale levels, each gray scale level having a differentoptical density, the High Energy Beam Sensitive-glass in bodies of 0.090inch cross section will exhibit the following properties:

[0086] (a) a transmittance of more than 88% at 436 nm; and

[0087] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm², said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂ 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0088] The present invention is also directed to a method of producingthree dimensional microstructures in substrate material comprisingexposing a substrate through a developed analog photoresist with a threedimensional microstructure with an ion beam in an ion beam etchingsystem to transfer the three dimensional microstructure of the developedanalog photoresist on to the surface of the substrate in a single stepexposure; the analog photoresist with three dimensional microstructurebeing the product of the process comprising exposing a photoresist to agray scale pattern in a gray scale mask using an optical lithographytool and developing the exposed photoresist to form three dimensionalmicrostructures in the photoresist; the gray scale mask comprising atransparent High Energy Beam Sensitive-glass (HEBS-glass) having atleast one gray scale zone with a plurality of gray scale levels, eachgray scale level having a different optical density, the High EnergyBeam Sensitive-glass in bodies of 0.090 inch cross section will exhibitthe following properties:

[0089] (a) a transmittance of more than 88% at 436 nm; and

[0090] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm², said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0091] The present invention is directed to a component having a threedimensional microstructure selected from the group consisting of taperedstructures for microelectronics, micro-optical devices, integratedoptical components, micro-electro-mechanical devices,micro-opto-electro-mechanical devices, microelectrical devices,diffractive optical elements (DOE), refractive microlens arrays,micromirror arrays, and diffractive microlens arrays; the componentcomprising a substrate having a three dimensional microstructureproduced by exposing a substrate through a developed analog photoresistwith a three dimensional microstructure with an ion beam in an ion beametching system to transfer the three dimensional microstructure of thedeveloped analog photoresist on to the surface of the substrate in asingle step exposure; the analog photoresist with three dimensionalmicrostructure being the product of the process comprising exposing aphotoresist to a gray scale pattern in a gray scale mask using anoptical lithography tool and developing the exposed photoresist to formthree dimensional microstructures in the photoresist; the gray scalemask comprising a transparent High Energy Beam Sensitive-glass(HEBS-glass) having at least one gray scale zone with a plurality ofgray scale levels, each gray scale level having a different opticaldensity, the High Energy Beam Sensitive-glass in bodies of 0.090 inchcross section will exhibit the following properties:

[0092] (a) a transmittance of more than 88% at 436 nm; and

[0093] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 mn to 630 mn per electrondosage unit of milli coulomb/cm², said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0094] The present invention is also directed to a component having athree dimensional microstructure selected from the group consisting ofelectrical connections between two metallic layers separated by taperedstructures of thick polyimide, bifocal intraocular lenses, widelyasymmetric DOE, random phase plate DOEs, miniature compact disc heads,antireflective surface, complex imaging optics, grating couples,polarization-sensitive beam splitters, spectral filters, wavelengthdivision multiplexers, micro optical elements for head-up and helmetmounted display, micro optical elements for focal plane opticalconcentration and optical efficiency enhancement, micro optical elementsfor color separation, beam shaping, and for miniature optical scanners,microlens arrays, diffraction gratings, diffractive lenses, laser diodearray collimators and correctors, micro optical elements for aberrationcorrection, hybrid optics, microprism arrays, micromirror arrays, randomphase plates and Bragg gratings, two dimensional fanout gratings,optical interconnects, signal switches, fiber pig tailing, DOEs forcoupling laser light into a fiber, micro-electro-mechanical sensors andactuators, micro valves, inertial micro sensors, micro machined RFswitches, GPS component miniaturization devices, laser scanners, opticalshutters, dynamic micro mirrors, optical choppers and optical switches;the component comprising a substrate having a three dimensionalmicrostructure produced by exposing a substrate through a developedanalog photoresist with a three dimensional microstructure with an ionbeam in an ion beam etching system to transfer the three dimensionalmicrostructure of the developed analog photoresist on to the surface ofthe substrate in a single step exposure; the analog photoresist withthree dimensional microstructure being the product of the processcomprising exposing a photoresist to a gray scale pattern in a grayscale mask using an optical lithography tool and developing the exposedphotoresist to form three dimensional microstructures in thephotoresist; the gray scale mask comprising a transparent High EnergyBeam Sensitive-glass (HEBS-glass) having at least one gray scale zonewith a plurality of gray scale levels, each gray scale level having adifferent optical density, the High Energy Beam Sensitive-glass inbodies of 0.090 inch cross section will exhibit the followingproperties:

[0095] (a) a transmittance of more than 88% at 436 mn; and

[0096] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm²; said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0097] The present invention is directed to a method of producing acomponent having a three dimensional microstructure selected from thegroup consisting of tapered structures for microelectronics, microoptical devices, integrated optical components, micro-electro-mechanicaldevices, micro-opto-electro-mechanical devices, diffractive opticalelements, refractive microlens arrays, diffractive microlens, andmicromirror arrays, the method comprising exposing a substrate through adeveloped analog photoresist with a three dimensional microstructurewith an ion beam in an ion beam etching system to transfer the threedimensional microstructure of the developed analog photoresist on to thesurface of the substrate in a single step exposure; the analogphotoresist with three dimensional microstructure being the product ofthe process of exposing a photoresist to a gray scale pattern in a grayscale mask using an optical lithography tool and developing the exposedphotoresist to form three dimensional microstructures in thephotoresist; the gray scale mask comprising a transparent High EnergyBeam Sensitive-glass (HEBS-glass) having at least one gray scale zonewith a plurality of gray scale levels, each gray scale level having adifferent optical density, the High Energy Beam Sensitive-glass inbodies of 0.090 inch cross section will exhibit the followingproperties:

[0098] (a) a transmittance of more than 88% at 436 nm; and

[0099] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm²; said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0100] The present invention is also directed to a method of producing acomponent having a three dimensional microstructure selected from thegroup consisting of electrical connections between two metallic layersseparated by tapered structures of thick polyimide, bifocal intraocularlenses, widely asymmetric DOE, miniature compact disc heads,antireflective surface, complex imaging optics, grating couples,polarization-sensitive beam splitters, spectral filters, wavelengthdivision multiplexers, micro optical elements for head-up and helmetmounted display, micro optical elements for focal plane opticalconcentration and optical efficiency enhancement, micro optical elementsfor color separations, beam shaping, and for miniature optical scanners,microlens arrays, diffraction gratings, diffractive lenses, laser diodearray collimators and correctors, micro optical elements for aberrationcorrection, hybrid optics, microprism arrays, micromirror arrays, randomphase plates and Bragg gratings, two dimensional fanout gratings,optical interconnects, signal switches, fiber pig tailing, DOEs forcoupling laser light into a fiber, micro-electro-mechanical sensors andactuators, micro valves, inertial micro sensors, micro machined RFswitches, GPS component miniaturization devices, laser scanners, opticalshutters, dynamic micro mirrors, optical shoppers and optical switches;the microlens, and micromirror arrays, the method comprising exposing asubstrate through a developed analog photoresist with a threedimensional microstructure with an ion beam in an ion beam etchingsystem to transfer the three dimensional microstructure of the developedanalog photoresist on to the surface of the substrate in a single stepexposure; the analog photoresist with three dimensional microstructurebeing the product of the process of exposing a photoresist to a grayscale pattern in a gray scale mask using an optical lithography tool anddeveloping the exposed photoresist to from three dimensionalmicrostructures in the photoresist; the gray scale mask comprising atransparent High Energy Beam Sensitive-glass (HEBS-glass) having atleast one gray scale zone with a plurality of gray scale levels, eachgray scale level having a different optical density, the High EnergyBeam Sensitive-glass in bodies of 0.090 inch cross section will exhibitthe following properties:

[0101] (a) a transmittance of more than 88% at 436 nm; and

[0102] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeof from 0.1 to 0.4 micrometer, and a value of beam current selected from25 to 250 na, an electron beam darkening sensitivity in the linearportion of the sensitivity curve, of at least 2.454 unit of opticaldensity value in the spectral range of 365 nm to 630 nm per electrondosage unit of milli coulomb/cm²; said HEBS-glass having a base glasscomposition consisting essentially on the mole % oxide basis 11.4 to17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%being TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to6% Cl; and 58.2 to 78.8% SiO₂.

[0103] The present invention is also directed to a Laser DirectWrite-glass (LDW-glass) which is a High Energy Beam Sensitive-glass(HEBS-glass) having at least a portion uniformly darkened to a uniformoptical density, said LDW-glass prior to being darkened with an electronbeam is a transparent HEBS-glass which in bodies 0.090 inch crosssection will exhibit the following properties:

[0104] (a) transmittance of more than 88% at 436 nm; and

[0105] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂.

[0106] The present invention is also directed to a gray scale mask on aLaser Direct Write glass (LDW-glass) produced by darkening at least aportion of a High Energy Beam Sensitive-glass (HEBS-glass) with anelectron beam to form a LDW-glass having a uniformly darkened portionhaving a uniform optical density, the HEBS-glass in bodies of 0.090 inchcross section exhibiting the following properties:

[0107] (a) transmittance of more than 88% at 436 nm; and

[0108] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0109] In an alternative embodiment of the gray scale mark, the grayscale zone has a continuous gray scale comprising a plurality of gradescale levels.

[0110] The present invention is also directed to a method of making agray scale mask comprising darkening at least a portion of a High EnergyBeam Sensitive-glass (HEBS-glass) with an electron beam to form a LaserDirect Write-glass having uniformly darkened portion having a uniformoptical density, the HEBS-glass in bodies of 0.090 inch cross sectionexhibiting the following properties:

[0111] (a) transmittance of more than 88% at 436 nm; and

[0112] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0113] In one embodiment of the method, the focused laser beam exposurewrite time for each area exposed is different. In another embodiment ofthe method, the focused laser beam intensity for each area exposed isdifferent. In still another embodiment of the method, the number ofretraces of the focused laser beam writing for each area exposed isdifferent.

[0114] The present invention is also directed to a method of making athree dimensional microstructure with three dimensional surfaces in aphotoresist comprising exposing a photoresist to a gray scale pattern ina gray scale mask on a Laser Direct Write-glass ((LDW-glass) using anoptical lithography tool and developing the exposed photoresist to formthree dimensional microstructures in the photoresist; A gray scale maskon a Laser Direct Write glass (LDW-glass) produced by darkening at leasta portion of a High Energy Beam Sensitive-glass (HEBS-glass) with anelectron beam to form a LDW-glass having a uniformly darkened portionhaving a uniform optical density, the HEBS-glass in bodies of 0.090 inchcross section exhibiting the following properties:

[0115] (a) transmittance of more than 88% at 436 nm; and

[0116] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0117] The present invention is also directed to an analog photoresistwith a three dimensional microstructure produced by exposing aphotoresist to a gray scale pattern in a gray scale mask on a LaserDirect Write-glass (LDW-glass) using an optical lithography tool anddeveloping the exposed photoresist to form the three dimensionalmicrostructure in the photoresist; the gray scale mask comprising:

[0118] A gray scale mask on a Laser Direct Write glass (LDW-glass)produced by darkening at least a portion of a High Energy BeamSensitive-glass (HEBS-glass) with an electron beam to form a LDW-glasshaving a uniformly darkened portion having a uniform optical density,the HEBS-glass in bodies of 0.090 inch cross section exhibiting thefollowing properties:

[0119] (a) transmittance of more than 88% at 436 nm; and

[0120] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0121] The present invention is directed to a method of producing threedimensional microstructures in substrate material comprising exposing asubstrate through a developed analog photoresist with a threedimensional microstructure with an ion beam in an ion beam etchingsystem to transfer the three dimensional microstructure of the developedanalog photoresist on to the surface of the substrate in a single stepexposure; the analog photoresist with three dimensional microstructurebeing the product of the process comprising exposing a photoresist to agray scale pattern in a gray scale mask on a Laser Direct Write-glass(LDW-glass) using an optical lithography tool and developing the exposedphotoresist to form three dimensional microstructures in thephotoresist; the gray scale mask comprising a LDW-glass having at leastone gray scale zone with a plurality of gray scale levels, each grayscale level having a different optical density, the gray scale maskproduced by darkening at least a portion of a High Energy BeamSensitive-glass (HEBS-glass) with an electron beam to form a LDW-glasshaving a uniformly darkened portion having a uniform optical density,the HEBS-glass in bodies of 0.090 inch cross section exhibiting thefollowing properties:

[0122] (a) transmittance of more than 88% at 436 nm; and

[0123] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0124] The present invention is directed to a component having a threedimensional microstructure selected from the group consisting of taperedstructures for microelectronics, micro-optical devices, integratedoptical components, micro-electro-mechanical devices,micro-opto-electro-mechanical devices, microelectrical devices,diffractive optical elements (DOE), refractive microlens arrays,micromirror arrays, and diffractive microlens arrays; the componentcomprising a substrate having a three dimensional microstructureproduced by exposing a substrate through a developed analog photoresistwith a three dimensional microstructure with an ion beam in an ion beametching system to transfer the three dimensional microstructure of thedeveloped analog photoresist on to the surface of the substrate in asingle step exposure; the analog photoresist with three dimensionalmicrostructure being the product of the process comprising exposing aphotoresist to a gray scale pattern in a gray scale mask on a LaserDirect Write-glass (LDW-glass) using an optical lithography tool anddeveloping the exposed photoresist to form three dimensionalmicrostructures in the photoresist; the gray scale mask comprising aLDW-glass having at least one gray scale zone with a plurality of grayscale levels, each gray scale level having a different optical density,the gray scale mask produced by darkening at least a portion of a HighEnergy Beam Sensitive-glass (HEBS-glass) with an electron beam to form aLDW-glass having a uniformly darkened portion having a uniform opticaldensity, the HEBS-glass in bodies of 0.090 inch cross section exhibitingthe following properties:

[0125] (a) transmittance of more than 88% at 436 nm; and

[0126] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0127] The present invention is directed to a component having a threedimensional microstructure selected from the group consisting ofelectrical connections between two metallic layers separated by taperedstructures of thick polyimide, bifocal intraocular lenses, widelyasymmetric DOE, miniature compact disc heads, antireflective surface,complex imaging optics, grating couples, polarization-sensitive beamsplitters, spectral filters, wavelength division multiplexers, microoptical elements for head-up and helmet mounted display, micro opticalelements for focal plane optical concentration and optical efficiencyenhancement, micro optical elements for color separation, beam shaping,and for miniature optical scanners, microlens arrays, diffractiongratings, diffractive lenses, laser diode array collimators andcorrectors, micro optical elements for aberration correction, hybridoptics, microprism arrays, micromirror arrays, random phase plates andBragg gratings, two dimensional fanout gratings, optical interconnects,signal switches, fiber pig tailing, DOEs for coupling laser light into afiber, micro-electro-mechanical sensors and actuators, micro valves,inertial micro sensors, micro machined RF switches, GPS componentminiaturization devices, laser scanners, optical shutters, dynamic micromirrors, optical choppers and optical switches; the component comprisinga substrate having a three dimensional microstructure produced byexposing a substrate through a developed analog photoresist with a threedimensional microstructure with an ion beam in an ion beam etchingsystem to transfer the three dimensional microstructure of the developedanalog photoresist on to the surface of the substrate in a single stepexposure; the analog photoresist with three dimensional microstructurebeing the product of the process comprising exposing a photoresist to agray scale pattern in a gray scale mask on a Laser Direct Write-glass(LDW-glass) using an optical lithography tool and developing the exposedphotoresist to form three dimensional microstructures in thephotoresist; the gray scale mask comprising a transparent High EnergyBeam Sensitive-glass having at least one gray scale zone with aplurality of gray scale levels, each gray scale level having a differentoptical density, the gray scale produced by darkening at least a portionof a High Energy Beam Sensitive-glass (HEBS-glass) with an electron beamto form a LDW-glass having a uniformly darkened portion having a uniformoptical density, the HEBS-glass in bodies of 0.090 inch cross sectionexhibiting the following properties:

[0128] (a) transmittance of more than 88% at 436 nm; and

[0129] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0130] The present invention is directed to a method of producing acomponent having a three dimensional microstructure selected from thegroup consisting of tapered structures for microelectronics, microoptical devices, integrated optical components, micro-electro-mechanicaldevices, micro-opto-electro-mechanical devices, diffractive opticalelements, refractive microlens arrays, diffractive microlens, andmicromirror arrays, the method comprising exposing a substrate through adeveloped analog photoresist with a three dimensional microstructurewith an ion beam in an ion beam etching system to transfer the threedimensional microstructure of the developed analog photoresist on to thesurface of the substrate in a single step exposure; the analogphotoresist with three dimensional microstructure being the product ofthe process of exposing a photoresist to a gray scale pattern in a grayscale mask on a Laser Direct Write-glass (LDW-glass) using an opticallithography tool and developing the exposed photoresist to form threedimensional microstructures in the photoresist; the gray scale maskcomprising a transparent High Energy Beam Sensitive-glass having atleast one gray scale zone with a plurality of gray scale levels, eachgray scale level having a different optical density, the gray scale maskproduced by darkening at least a portion of a High Energy BeamSensitive-glass (HEBS-glass) with an electron beam to form a LDW-glasshaving a uniformly darkened portion having a uniform optical density,the HEBS-glass in bodies of 0.090 inch cross section exhibiting thefollowing properties:

[0131] (a) transmittance of more than 88% at 436 nm; and

[0132] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0133] The present invention is also directed to a method of producing acomponent having a three dimensional microstructure selected from thegroup consisting of electrical connections between two metallic layersseparated by tapered structures of thick polyimide, bifocal intraocularlenses, widely asymmetric DOE, miniature compact disc heads,antireflective surface, complex imaging optics, grating couples,polarization-sensitive beam splitters, spectral filters, wavelengthdivision multiplexers, micro optical elements for head-up and helmetmounted display, micro optical elements for focal plane opticalconcentration and optical efficiency enhancement, micro optical elementsfor color separations, beam shaping, and for miniature optical scanners,microlens arrays, diffraction gratings, diffractive lenses, laser diodearray collimators and correctors, micro optical elements for aberrationcorrection, hybrid optics, microprism arrays, micromirror arrays, randomphase plates and Bragg gratings, two dimensional fanout gratings,optical interconnects, signal switches, fiber pig tailing, DOEs forcoupling laser light into a fiber, micro-electro-mechanical sensors andactuators, micro valves, inertial micro sensors, micro machined RFswitches, GPS component miniaturization devices, laser scanners, opticalshutters, dynamic micro mirrors, optical shoppers and optical switches;the microlens, and micromirror arrays, the method comprising exposing asubstrate through a developed analog photoresist with a threedimensional microstructure with an ion beam in an ion beam etchingsystem to transfer the three dimensional microstructure of the developedanalog photoresist on to the surface of the substrate in a single stepexposure; the analog photoresist with three dimensional microstructurebeing the product of the process of exposing a photoresist to a grayscale pattern in a gray scale mask on a Laser Direct Write-glass(LDW-glass) using an optical lithography tool and developing the exposedphotoresist to from three dimensional microstructures in thephotoresist; the gray scale mask comprising a transparent High EnergyBeam Sensitive-glass having at least one gray scale zone with aplurality of gray scale levels, each gray scale level having a differentoptical density, the gray scale mask produced by darkening at least aportion of a High Energy Beam Sensitive-glass (HEBS-glass) with anelectron beam to form a LDW-glass having a uniformly darkened portionhaving a uniform optical density, the HEBS-glass in bodies of 0.090 inchcross section will exhibit the following properties:

[0134] (a) transmittance of more than 88% at 436 nm; and

[0135] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, an addressing grid sizeselected from 0.1 to 0.4 micrometer, and a value of beam currentselected from 25 to 250 na, an electron beam darkening sensitivity inthe linear portion of the sensitivity curve of at least 2.454 unit ofoptical density value in the spectral range of 365 nm to 630 nm perelectron dosage unit of milli coulomb/cm², said HEBS-glass having a baseglass composition consisting essentially on the mole % oxide basis 11.4to 17.5% of one or more alkali metal oxide, 2.4 to 10.2% ofphotosensitivity inhibitors and RS suppressing agents with at least 2.4%TiO₂; 1.1 to 2.4% Al₂O₃; 0 to 4.6% B₂O₃; 3.7 to 13.2% ZnO; 0.5 to 6% Cl;and 58.2 to 78.8% SiO₂; and exposing a plurality of areas on theuniformly darkened portion of the LDW-glass with a focused laser beam toform a gray scale zone with a plurality of gray scale levels, theoptical density of each gray scale level differing from the opticaldensity of adjacent gray scale levels, and the optical density of thedarkest gray scale level not exceeding the optical density of theuniformly darkened portion of the LDW-glass.

[0136] A method of fabricating a three-dimensional micro-optic lens on asubstrate selected from a group consisting of quartz glass, silicateglass, germanium and an optically transmissive material coated with aphotoresist layer, comprising: providing a gray scale mask having a bodyportion and a surface layer formed thereon which is responsive toelectron beam radiation to change the optical density of the surfacelayer; exposing the mask to an electron beam of selected charge densityover a grid of discrete locations on the mask to provide a predeterminedgray scale pattern of continuously varying optical transmissivity on themask; exposing the photoresist layer to radiation transmitted throughthe mask; and removing material from the photoresist layer and thesubstrate to provide a predetermined varying thickness of the substrateas determined by the gray scale patterns on the mask.

[0137] The above method optionally including the step of generating saidelectron beam with a current of at least about 25 nA.

[0138] The above method optionally including the step of applying anelectrically conductive coating to the mask prior to exposing the maskto said electron beam and removing said coating from the mask afterexposing the mask to said electron beam.

[0139] The above method optionally including the step of removingmaterial from said photoresist layer and said substrate by chemicallyassisted ion beam etching.

[0140] The above method optionally including the step of comparing athickness of said photoresist layer which may be exposed to radiationwith a corresponding electron beam charge density value required todarken said layer of the mask to provide a predetermined depth level insaid substrate; and exposing the mask to said electron beam at apreselected charge density corresponding to the desired thickness ofexposure of said photoresist layer.

[0141] Another method for producing various depth levels in a layer ofphotoresist material includes the steps of exposing a layer ofphotoresist material to radiation through a gray scale mask having areasof continuously varying transmissivity; removing photoresist materialfrom said photoresist layer to depth in said photoresist layer at apredetermined position thereon corresponding to a predeterminedtransmissivity of said gray scale Mask at a corresponding predeterminedposition on said gray scale mask; and providing said gray scale mask asa glass article comprising a body portion and an integral ion exchangedsurface layer which, upon exposure to a high energy electron beam,becomes darkened and is substantially insensitive to actinic radiation.

[0142] The above method optionally including the step of exposing saidgray scale mask to selected discrete charge densities of electron beamradiation over a grid of preselected grid spacings and varying theelectron beam charge density from one spacing to the next in accordancewith a predetermined depth level desired to be produced in saidphotoresist layer.

[0143] The above method optionally including the step of comparing athickness of said photoresist layer which may be exposed to radiationwith a corresponding electron beam charge density value required todarken said gray scale mask to provide a predetermined depth level insaid photoresist layer; and exposing said gray scale mask to saidelectron beam at a preselected charge density corresponding to thedesired thickness of exposure of said photoresist layer.

[0144] The above method optionally including the step of selectivelydarkening a surface layer of said gray scale mask by generating anelectron beam at discrete, predetermined positions thereon and at anacceleration voltage of at least about 20 kV.

[0145] Another method of fabricating a three-dimensional micro-elementon a substrate to various depth levels comprising one of discrete depthlevels and a continuous depth profile through a photoresist layer,comprises the steps of exposing said photoresist layer to radiationtransmitted through a gray scale mask having a gray scale patternthereon comprising image areas having a continuously varyingtransmissivity corresponding to a depth of material to be removed fromsaid substrate to provide said element; removing material from saidphotoresist layer and said substrate in a predetermined pattern asdetermined by said gray scale pattern on said mask; providing said grayscale mask characterized as a glass article comprising a body portionand an integral radiation absorbing surface layer which is substantiallyinsensitive to actinic radiation; and providing said glass article withsaid ion exchanged surface layer having Ag+ ions therein, and/or silverhalide containing and/or Ag.sub.2 O containing and/or Ag+ ion containingmicro-crystals and/or micro-phases therein.

[0146] The above method optionally including the step of exposing themask to an electron beam at a predetermined dosage corresponding to adegree of darkening of the mask required to produce a predetermineddepth level in said photoresist layer.

[0147] The above method optionally including the step of darkening themask by generating an electron beam at an acceleration voltage in therange of 20 kV to 30 kV.

[0148] The above method optionally including the step of exposing themask to an electron beam charge density of 0 mC/cm.sup.2 to about 400mC/cm.sup.2.

[0149] The above method optionally including the step of generating saidelectron beam with a current of at least about 25 nA.

[0150] The above method optionally including the step of applying anelectrically conductive coating to the mask prior to exposing the maskto said electron beam.

[0151] The above method optionally including the step of removing saidcoating from the mask after exposing the mask to said electron beam.

[0152] The above method optionally including the step of comparing athickness of said photoresist layer which may be exposed to radiationwith a corresponding electron beam charge density value required todarken the mask to provide a predetermined depth level in saidsubstrate; and exposing the mask to said electron beam at a preselectedcharge density corresponding to the desired thickness of exposure ofsaid photoresist layer.

[0153] The above method optionally including the step of: exposing themask to selected discrete charge densities of electron beam radiationover a grid of preselected grid spacings and varying the electron beamcharge density from one spacing to the next in accordance with apredetermined depth level desired to be produced in said substrate.

[0154] Another method of fabricating a three-dimensional micro-elementon a substrate and to various depth levels comprising one of discretedepth levels and a continuously depth profile through a photoresistlayer, comprises the steps of providing a gray scale mask characterizedas a glass article comprising a body portion and an internal radiationabsorbing surface layer which is substantially insensitive to actinicradiation; and providing said glass article as a silicate glass having asilicon dioxide content in mole percent of from 30 to 95 and essentiallyno transition metals having 1-4 d electrons in the atomic state and atleast one surface of said article having a substantially continuoussilver and hydration content over its area, effective to render saidsurface darkenable upon exposure to electron beam radiation.

[0155] The present invention provides an improved method for producingmicro-elements, including diffractive optical elements and the like,using a gray scale mask.

[0156] The present invention also provides an improved method forproducing a gray scale mask comprising a glass article which issensitive to exposure to a high energy electron beam, for example, toprovide a pre-determined pattern on the article by varying the opticaldensity of the glass as a result of exposure to the high energy electronbeam.

[0157] In accordance with one aspect of the present inventionmicro-elements, such as diffractive optical elements, computer-generatedholograms and other three dimensional micro-elements, may be producedwith greater accuracy of the prescribed geometry of the element and inlarge quantities or large arrays by providing a gray scale mask having amasking pattern developed on a durable glass substrate comprising a highenergy beam sensitive (HEBS) glass. The glass substrate or articleincludes a body portion and an integral ion exchanged surface layerwhich, upon exposure to high energy electron beams, becomes darkened toa selected degree to provide the gray levels required for developing thevarious depths or phase levels in the three dimensional elements to bemanufactured using the mask. In particular, the mask glass articlepreferably comprises a plate of a high energy beam-sensitive glasshaving an integral ion exchanged surface layer containing a highconcentration of Ag.sup.+ ions and/or a large number density ofAgCl-containing and/or Ag.sub.2O-containing and/or Ag.sup.+ion-containing micro-crystals and/or micro-phases, and also containingsilanol groups and/or water in the concentration range of about 0.01% to12.0% by weight water. The gray scale mask may also be formed of a glasssuch as a silicate glass composition hydrated and containing silver andwhich can be effectively written with high energy beams, such aselectron beams, to produce high optical density images thereon.

[0158] In accordance with another aspect of the present invention amethod for generating a gray scale mask is provided wherein a glass maskelement is provided with a pre-determined masking pattern formeddirectly thereon to provide a durable mask structure which eliminatesthe need for thin film coatings and ablative thin film materials. Theparticular method for producing a mask structure contemplated by thepresent invention comprises exposing a high energy beam-sensitive glassplate directly to an electron beam, using a commercially availableelectron beam writing device, at a relatively low acceleration voltageto provide a more precise configuration of the mask image pattern andthe variations of optical density required to generate the various graylevels. In particular, acceleration voltage is controlled to producesufficient penetration depth in the mask material without extending theelectron trajectories unnecessarily with the resultant loss inresolution of the mask pattern.

[0159] In accordance with still another aspect of the present inventionmicro-elements, such as diffractive optical elements, are fabricatedwith improved geometries using a gray scale mask formed of a glasscomposition which is operable to provide stable images generated byexposure to an electron beam which may be controlled to generate aprecise image on the glass. A gray scale mask in accordance with theinvention may be reused many times, is relatively insensitive toexposure to environmental factors and is capable of providing highresolution and the resultant precise contour or dimensional control overthe workpiece.

[0160] The present invention further provides an improved method offabricating micro-optic devices, such as diffractive optical elements,with a gray scale mask which is simplified and cost effective, andwherein only a single mask needs to be exposed in an electron beamwriter and wherein no multiple resist processing steps are required togenerate the mask. Since the multiple levels or contour shading of thegray levels are written in a single step on a single mask the inevitablemisregistrations between multiple lithography steps used in prior artmask fabricating methods are avoided.

[0161] Still further, the number of processing steps for fabrication ofmicro-elements compared to the steps required in fabrication methodsusing binary masks is substantially reduced in the method of the presentinvention wherein the element workpiece material may be optimized, thatis the material which is best suited for the application can be chosenwithout being limited by the constraints of a molding material used inmolded element fabrication methods.

[0162] Moreover, the method of the invention utilizes certain materials,tools and equipment compatible with the fabrication of large scaleintegrated electronic circuits. In this regard, the development of newfabrication techniques, environments and computer programs, for example,are not required to be established. The reduction in the number of stepsinvolved in the fabrication method of the present invention will improvethe efficiency and speed of the fabrication process. In this regard,mass production may be carried out based on a step and repeatphotoresist exposure process followed by a chemically assisted ion beametching batch process, for example.

[0163] Those skilled in the art will further appreciate theabove-mentioned advantages and superior features of the inventiontogether with other important aspects thereof upon reading the detaileddescription which follows in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0164]FIG. 1 illustrates a qualitative representation of the silverconcentration profile in HEBS-glass.

[0165]FIG. 2 records absorbence spectra of HEBS-glass No. 3 after floodexposure with e-beam at 29 kV acceleration voltage.

[0166]FIG. 3 records absorbence spectra of HEBS-glass No. 3 after floodexposure with e-beam at 25 kV acceleration voltage.

[0167]FIG. 4 records absorbence spectra of HEBS-glass No. 3 after floodexposure with e-beam at 20 kV acceleration voltage.

[0168]FIG. 5 records absorbence spectra of HEBS-glass No. 3 after floodexposure with e-beam at 15 kV acceleration voltage.

[0169]FIG. 6 depicts optical density at 436 nm of HEBS-glass No. 3versus electron dosage. Electron beam exposure was done with EVC floodexposure system at 29 kV,20 kV,and 15 kV.

[0170]FIG. 7(a) records net optical density at 435 nm versus electrondosage at 30 kV. Curve A—250 na, 0.4 μm address size, Curve B—75 na, 0.2μm address size, EVC—e-beam flood exposure.

[0171]FIG. 7(b) records net optical density at 530 nm versus electrondosage at 30 kV. Curve A—250 na, 0.4 μm address size, Curve B—75 na, 0.2μm address size, EVC—e-beam flood exposure.

[0172]FIG. 7(c) records net optical density at 630 nm versus electrondosage at 30 kV. Curve A—250 na, 0.4 μm address size, Curve B—75 na, 0.2μm address size, EVC—e-beam flood exposure.

[0173]FIG. 7(d) records data points of the net optical density at 365 nmvs. electron dosage, depicts the best fit curve and displays theequation describing the best fit curve; the electron beam exposure wasdone with Cambridge EBMF 10.5 e-beam writer operated at 30 kV having abeam current of 250 na and an addressing grid spacing of 0.4 μm. In theequation, Y represents the net optical density at 365 nm and Xrepresents values of electron dosage in milli-coulomb/cm².

[0174]FIG. 7(e) records data points of the net optical density at 435 nmvs. electron dosage, depicts the best fit curve and displays theequation describing the best fit curve; the electron beam exposure wasdone with Cambridge EBMF 10.5 e-beam writer operated at 30 kV having abeam current of 250 na and an addressing grid spacing of 0.4 μm. In theequation, Y represents the net optical density at 435 nm and Xrepresents values of electron dosage in milli-coulomb/cm².

[0175]FIG. 7(f) records data points of the net optical density at 530 nmvs. electron dosage, depicts the best fit curve and displays theequation describing the best fit curve; the electron beam exposure wasdone with Cambridge EBMF 10.5 e-beam writer operated at 30 kV having abeam current of 250 na and an addressing grid spacing of 0.4 μm. In theequation, Y represents the net optical density at 530 nm and Xrepresents values of electron dosage in milli-coulomb/cm².

[0176]FIG. 7(g) records data points of the net optical density at 630 nmvs. electron dosage, depicts the best fit curve and displays theequation describing the best fit curve; the electron beam exposure wasdone with Cambridge EBMF 10.5 e-beam writer operated at 30 kV having abeam current of 250 na and an addressing grid spacing of 0.4 μm. In theequation, Y represents the net optical density at 630 nm and Xrepresents values of electron dosage in milli-coulomb/cm².

[0177]FIG. 8 records net optical density at 436 nm versus electrondosage at 20 kV. Curve A—MEBES m, 4000 na, 0.5 μm address size, 40 MHz.Curve B—Cambridge EBMF 10.5, 25 na, 0.1 μm address size. EVC—e-beamflood exposure.

[0178]FIG. 9 depicts the transmittance spectra of exemplary HEBS-glassNo. 3. A—a base glass plate 0.090″ thick; B—a HEBS-glass plate 0.086″thick having one ion-exchanged surface glass layer; C—a HEBS-glass plate0.090″ thick having two ion-exchanged surface glass layer.

[0179]FIG. 10 exhibits an optical micrograph of a gray scale mask whichis a grating in HEBS-glass No. 3. The grating has 250 gray levels withina period of 200 μm.

[0180]FIG. 11 exhibits an optical micrograph of a portion of a grayscale mask which is a diffractive optical lens having ten gray levels ineach zone.

[0181]FIG. 12 illustrates that the processing steps necessary togenerate Diffractive Optical Elements consisting of (a) a HEBS-glassphoto mask blank being exposed in e-beam writer (b) gray level maskgenerated in HEBS-glass (c) photoresist exposure in optical lithographytool (d) resist surface profile after development (e) surface profile insubstrate material after CAIBE transfer step.

[0182]FIG. 13 illustrates that the processing steps to fabricaterefractive lens arrays consisting of (a) a HEBS-glass photo mask blankbeing exposed in e-beam writer (b) gray level mask generated inHEBS-glass (c) photoresist exposure in mask aligner (d) resist surfaceprofile after development (e) lens profile after etching transfer step.

[0183]FIG. 14 depicts the thickness of Shipley S1650 Photoresist versusoptical density at 436 nm of HEBS-glass mask; photoresist was exposed inan optical contact aligner.

[0184]FIG. 15 records the calibration curve “net optical density at 435nm versus clock rate” of e-beam exposure at 30 kv using Cambridge EBMF10.5 e-beam writer with 250 na beam current and 0.4 μm addressing gridsize.

[0185]FIG. 16 records the calibration curve “net optical density at 435nm versus clock rate” of e-beam exposure at 30 kv using Cambridge EBMF10.5 e-beam writer with 75 na beam current and 0.2 μm addressing gridsize.

[0186]FIG. 17 records the calibration curve “1/(clock rate) versus netoptical density at 435 nm” of e-beam exposure at 30 kv using CambridgeEBMF 10.5 e-beam writer with 250 na beam current and 0.4 μm addressinggrid size.

[0187]FIG. 18 records the calibration curve “1/(clock rate) versus netoptical density at 435 nm” of e-beam exposure at 30 kv using CambridgeEBMF 10.5 e-beam writer with 75 na beam current and 0.2 μm addressinggrid size.

[0188]FIG. 19 depicts absorption spectra of LDW-HR plates-Type I, -TypeII and -Type III.

[0189]FIG. 20 depicts absorption spectra of LDW-IR plates-Type I, -TypeII and Type III;

[0190]FIG. 21 is a diagram showing the variation in optical density of agray scale mask formed of high energy beam sensitive glass afterexposure with an electron beam of a particular acceleration voltage;

[0191]FIG. 22 is a diagram similar to FIG. 1 showing the variation inoptical density of a mask formed of the same material after exposure toa beam of higher acceleration voltage;

[0192]FIG. 23 is a diagram showing photoresist thickness versus electroncharge density or dosage used to expose a gray scale mask in accordancewith the present invention;

[0193]FIG. 24 is a transverse section view of a portion of a micro-lensfabricated in accordance with the method of the present invention;

[0194]FIG. 25A is a partial plat of a grid showing darkened areascorresponding to changes in contour of portions of the lens shown inFIG. 4;

[0195]FIG. 25B is intended to be read in conjunction with FIG. 5A andshows a partial transverse section on a larger scale of the micro-lensshown in FIG. 4;

[0196]FIG. 26 is a perspective view, greatly enlarged, of a gray scalemask and a photoresist coated substrate for fabricating an array ofmicro-lenses in accordance with the present invention; and

[0197]FIG. 27 is a diagram showing the geometry of a micro-lensfabricated in accordance with the method of the invention.

DESCRIPTION OF THE INVENTION

[0198] High energy beam sensitive glasses used to generate the graylevel mask, consist of a low expansion zinc-silicate glass, a whitecrown glass. The base glass can be produced from glass melting just likethe conventional white crown optical glasses. The base glass containsalkali to facilitate the following ion-exchange reactions which achievethe sensitivity of the HEBS-glass towards high energy beams, e-beam inparticular. After ion-exchange HEBS-glass is essentially alkali free asa result of the ion-exchange process and the concurrent leaching processcarried out in an acidic aqueous solution at temperatures above 320° C.The base glass composition consists of silica, metal oxides, halides andphoto inhibitors. Typically TiO₂, Nb2O₅ or Y₂O₃ are used as photoinhibitors. The photo inhibitors are used to dope the silver ioncontaining complex crystals, silver-alkali-halide. These (AgX)_(m)(MX)_(n) in complex crystals are the beam sensitive material and thedoping of the photo inhibitors increases the energy band gap of theotherwise photosensitive glass.

[0199] The exemplary glass compositions that are optimized for makingHEBS-glass gray level masks are listed in Exhibit A. Photosensitivityinhibitors and RS-suppression agents other than TiO₂, selected from thegroup consisting of Ta₂O₅, ZrO₂, Nb₂O₅, La₂O₃, Y₂O₃ and WO₃ mayoptionally be added to the glass batch or replaces portions of TiO₂ inthe base glass compositions of Exhibit A. More than about 1% of Cl isadded in the forms of Alkali Chloride to the glass batch to ensure thatthe glass melt is saturated with chlorides. The Chlorides also functionas a fining agent for the glass melt.

[0200] The base glass compositions of the present invention consist inthe glass batch essentially of, in mole percent on the oxide basis, 11.4to 17.5% of one or more alkali metal oxides, 2.4 to 10.2% total of photosensitivity inhibitors and RS suppression agents including 2.4 to 10.2%TiO₂, 1.1 to 2.4% Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl,and 58.2 to 78.8% SiO₂.

[0201] After the glass is melted, drawn, ground and polished the baseglass plates are ion-exchanged in an acidic aqueous solution containingsoluble ionic silver. The ion exchange process is carried out attemperatures in excess of 320° C. for a duration sufficient to causesilver ions to diffuse into the glass plates 3 μm, i.e., (x₂−x₁) in thethickness dimension of FIG. 1. As a result silver ions are present inthe form of silver-alkali-halide (AgX)_(m)(MX)_(n) complex crystals thatare about 10 nm in each dimension within the cavity of the SiO₄tetrahedron network.

[0202] Ground and polished glass plates of the exemplary glasscompositions of Exhibit A were ion exchanged in aqueous solutioncontaining ionic silver. The aqueous ion exchange solution consists, onthe weight percent basis, 7.5% or more of AgNO₃ and 0.5% or more ofHNO₃. HEBS-glass No. 1 to No. 20 are the glass plates of the exemplarybase glass compositions No. 1 to No.20 respectively having been ionexchanged in the aqueous ion exchange solution.

[0203] Doping of the base glass with the photo inhibitors causes anincreased energy band gap, making the ion exchanged glasses inert to UVand actinic radiation of shorter wavelengths as the concentration of thedoping with photo inhibitors increases. Nevertheless the chemicalreduction of silver ions in the silver-alkali-halide containing complexcrystals to produce coloring specks of silver atoms can be accomplishedby exposing the HEBS-glass to high energy beams, eg., >10 kV electronbeams. This property of the material can be utilized to generate thenecessary change in transmission for a gray level mask.

[0204] FIGS. 2 to 5 exhibit the resulting optical density of theHEBS-glass No. 3 of the exemplary base glass composition No. 3 afterexposure with a flood electron beam exposure system using a 29 kV, a 25kV, a 20 kV and a 15 kV electron beam respectively at a number of dosagelevels. The flood e-beam exposure system manufactured and marketed byEVC Corporation, San Diego, Calif., has a beam diameter of 8 inches andwas operated at a beam current of 2 milli amp. The absorption data wascollected using a Hitachi U2000 spectrophotometer.

[0205] Optical density values of HEBS-glass No. 3 at 436 nm as afunction of e-beam dosage is plotted in FIG. 6 for e-beam accelerationvoltages of 29 kV, 20 kV and 15 kV. In this plot the finite opticaldensity value at zero electron dosage is due to reflection loss ofprobing light beam at two surfaces of glass plate samples. To Exhibit AExemplary Glass Compositions GLASS NO. 1 2 3 4 5 6 7 8 9 10 SiO₂ 71.578.8 68.5 72.7 70.9 68.9 67.4 67.1 66.1 63.8 Li₂O 3.3 3.4 3.8 3.6 3.73.9 3.9 4.2 4.2 4.5 Na₂O 5.3 5.4 6.4 5.7 5.6 6.2 6.2 6.7 6.7 7.2 K₂O 2.82.7 3.2 3.1 3.1 3.3 3.3 3.5 3.5 3.8 TiO₂ 2.4 4.3 4.6 3.4 4.5 5.6 4.5 5.45.4 6.7 Al₂O₃ 1.3 1.2 1.3 1.4 1.2 1.1 1.3 1.2 1.1 1.6 ZnO 7.2 3.7 7.47.1 6.0 7.0 9.0 7.1 8.2 7.6 Ta₂O₅ Nb₂O₃ ZrO₂ WO₃ B₂O₃ 3.2 1.8 2.0 1.01.4 1.8 0.8 1.8 Cl 3.0 0.5 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 GLASS NO. 1112 13 14 15 16 17 18 19 20 SiO₂ 64.8 64.0 60.1 60.5 58.2 69.7 64.2 64.566.3 67.8 Li₂O 4.5 4.7 4.3 5.1 5.1 3.9 3.8 3.8 3.8 3.8 Na₂O 7.4 7.6 7.88.1 8.1 6.2 6.4 6.4 6.4 6.4 K₂O 3.6 4 4.2 4.3 4.3 3.3 3.2 3.2 3.2 3.2TiO₂ 5.4 7.4 6.1 10.2 5.7 4.4 4.6 4.6 4.6 4.6 Al₂O₃ 1.2 1.2 1.5 1.2 2.41.2 1.3 1.3 1.3 1.3 ZnO 10.1 8.1 11.0 7.1 13.2 7.1 7.4 7.4 7.4 7.4 Ta₂O₅1.6 Nb₂O₃ 1.2 ZrO₂ 2.0 WO₃ 0.5 B₂O₃ 1.8 2.0 0.5 2.0 4.6 2.0 2.0 Cl 1.23.0 3.0 3.0 3.0 2.2 6.0 3.0 3.0 3.0

[0206] obtain an optical density value of 1.0 at 436 nm in HEBS-glassthe required electron dosage is 75 μC/cm², 155 μC/cm² and 270 μC/cm²using EVC e-beam exposure system at 29 kV, 20 kV and 15 kV respectively.

[0207] The e-beam exposure-induced optical density i.e. net opticaldensity in HEBS-glass is a function of e-beam exposure scheme and writeparameters which include e-beam energy (i.e. e-beam accelerationvoltage), beam spot size, beam current and addressing grid. The netoptical density is defined herein as the optical density of the e-beamdarkened area minus the optical density of the clear (unexposed) area.

[0208] The net optical density in the visible spectral range wasmeasured as a function of electron dosage using HEBS-glass No. 3 havingbeen exposed in a number of 3 mm×3 mm square areas with the followinge-beam pattern generators:

[0209] (1) MEBES of ETEC Systems, Inc., (2) Cambridge EBMF 10.5 e-beamwriter. Results of exemplary exposure schemes are discussed immediatelybelow.

[0210]FIG. 7(a) exhibits net optical density values of HEBS-glass No. 3at 435 nm vs. electron dosage at 30 kv. The e-beam exposure was doneusing the vector scan e-beam writer, Cambridge EBMF 10.5. The e-beamparameters are as follows:

[0211] Curve A—30 kv, 250 na beam current, 0.4μm addressing gridspacing.

[0212] Curve B—30 kv, 75 na beam current, 0.2 μn addressing gridspacing.

[0213] The data points of the net optical density values at 435 nmresulting from EVC flood gun exposure at 30 kV are shown in the figurefor comparison.

[0214]FIG. 7(b) displays the corresponding net optical density values at530 nm as a function of electron dosage at 30 kv.

[0215]FIG. 7(c) exhibits the corresponding net optical density values at630 nm as a function of electron dosage at 30 kv.

[0216] The data points of curve A in FIGS. 7 are listed in Table 1. Alsolisted in Table 1 is the net optical density values at 365 nm. Theelectron dosage in μ Coulomb/cm² and in milli Coulomb/cm², the clockrates and the corresponding e-beam exposure durations per address toresult in the tabulated electron dosages are also listed in the table.

[0217] The best fit polynomial equations that depict the experimentaldata of net optical density vs. electron dosage in milli coulomb/cm² ofTable 1 are shown in FIG. 7(d), FIG. 7(e), FIG. 7(f) and FIG. 7(g)respectively for the net optical density values at 365 nm, 435 mm, 530nm and 630 nm respectively. In the equations, y represents net opticaldensity values and x represents values of electron dosage in millicoulomb/cm². The experimental data points and the best fitted curves arealso shown in FIGS. 7(d), 7(e), 7(f), and 7(g).

[0218] As shown in FIG. 7(d), a large portion of the best fit curve is astraight line. The linear portion ranges in net optical density valuesfrom 0 to 0.9. The slope of the linear portion representing the e-beamsensitivity of HEBS-glass No. 3 darkening at the spectral wavelength of365 nm upon e-beam exposure with write parameters of 30 kV accelerationvoltage, 250 nano-amp beam current, and 0.4 μm addressing grid size, is6.2767 unit of optical density value per milli coulomb/cm². Namely toobtain a net optical density value of 0.62767 at 365 nm, the requiredelectron dosage is 100 micro-coulomb/cm².

[0219] As shown in FIG. 7(e), a large portion of the best fit curve is astraight line. The linear portion ranges in net optical density valuesfrom 0 to 1.65. The slope of is 1 the straight line portion representingthe e-beam sensitivity of HEBS-glass No. 3 darkening at the spectralwavelength of 435 nm upon e-beam exposure with write parameters of 30 kVacceleration voltage, 250 nano-amp beam current, and 0.4 μm addressinggrid size, is 9.2113 unit of optical density value per millicoulomb/cm². Namely to obtain a net optical density value of 0.92113 at435 nm, the required electron dosage is 100 micro-coulomb/cm².

[0220] As shown in FIG. 7(f), a large portion of the best fit curve is astraight line. The linear portion ranges in net optical density valuesfrom 0 to 2.05. The slope of the linear portion representing the e-beamsensitivity of HEBS-glass No. 3 darkening at the spectral wavelength of530 nm upon e-beam exposure with write parameters of 30 kV accelerationvoltage, 250 nano-amp beam current, and 0.4 μm addressing grid size, is12.507 unit of optical density value per milli coulomb/cm². Namely toobtain a net optical density value of 1.2507 at 530 nm, the requiredelectron dosage is 100 micro-coulomb/cm².

[0221] As shown in FIG. 7(g), a large portion of the best fit curve is astraight line. The linear portion ranges in net optical density valuesfrom 0 to 1.7. The slope of the linear portion representing the e-beamsensitivity of HEBS-glass No. 3 darkening at the spectral wavelength of630 nm upon e-beam exposure with write parameters of 30 kV accelerationvoltage, 250 nano-amp beam current, and 0.4 μm addressing grid size, is9.5929 unit of optical density value per milli coulomb/cm². Namely toobtain a net optical density value of 0.95929 at 630 nm, the requiredelectron dosage is 100 micro-coulomb/cm².

[0222] Electron beam pattern generators were employed to darkenHEBS-glass No. 3 at a number of electron dosage levels using beamacceleration voltages, beam current, beam spot size and addressing gridsize as variable parameters. Beam acceleration voltages ranging from 10kV to 50 kV, beam spot size ranging from 0.1 μm to 1 μm, beam currentranging from 10 na to 8000 na, and addressing grid size ranging from0.05 μm to 1 μm were studied to determine the practical and costeffective write schemes for HEBS-glass compositions. Experimental dataof net optical density in the spectral range of 350 nm to 1100 nm andpolynomial equations together with the best fit curves resembling FIGS.7 (d) to 7(g) as well as the slope of the linear portion of the best fitcurves were obtained for a number of combinations of e-beam writerparameters. It has been determined that the exemplary write schemesusing EBMF 10.5 e-beam writer, which are practical and cost effective tomake HEBS-glass gray level masks include (1) 30 kV, 0.4 μm address, 250na.; (2) 30 kV, 0.2 μm address, 150 na; (3) 30 kV, 0.2 μm address, 125na; (4) 30 kV, 0.2 μm address, 100 na; (5) 30 kV, 0.2 μm address, 75 na;(6) 20 kV, 0.2 μm address, 175 na; (7) 20 kV, 0.2 μm address, 150 na;(8) 20 kV, 0.2 μm address, 125 na; (9) 20 kV, 0.2 μm address, 100 na;and (10) 20 kV, 0.1 μm address, 25 na. Using each of the ten writeschemes listed immediately above net optical density values ofHEBS-glass NO.3 at wavelengths from 350 nm to 1100 nm were obtained as afunction of the electron dosage. The best fit polynomial equations andthe slope of the linear portion of each of the best fit curves of netoptical density values at 365 nm, at 435 nm, at 530 nm and 630 nm vs.electron dosage are represented in Table 2.

[0223]FIG. 8 exhibits net optical density values of HEBS-glass No. 3 at436 nm vs. electron dosage at 20 kv. Curve A displays the data of e-beamexposure using the raster scan e-beam pattern generator, MEBES m. MEBESm was operated at 20 kv. 40 MHz rate, using a spot size of 1 μm, a beamcurrent of 4000 na and an addressing grid size of 0.5 μm. These writeparameters result in an exposure dosage of 40 μC/cm² per scan count.Electron dosage having multiples of 40 μC /cm² were

[0224] Table 1: Net optical density in HEBS-glass having been exposed toe-beam at 30 kv, 250 na beam current, 0.4 μm addressing grid size usingCambridge EBMF 10.5 e-beam writer at various clock rates. TABLE 1 Netoptical density in HEBS-glass having been exposed to e-beam at 30 kv,250 na beam current, 0.4 μm addressing grid size using Cambridge EBMF10.5 e-beam writer at various clock rates. Exposure Net Net Net NetDuration Optical Optical Optical Optical per pixel Clock Rate ElectronDosage Density at Density at Density at Density at (micro sec) (MHz)(micro C/cm²) (milli C/cm²) 365 nm 435 nm 530 nm 630 nm 0.1039 9.62516.23 0.01623 0.170 0.172 0.149 0.100 0.2091 4.782 32.67 0.03267 0.2750.317 0.336 0.242 0.2984 3.351 46.63 0.04663 0.362 0.445 0.514 0.3770.4126 2.424 64.47 0.06447 0.472 0.613 0.744 0.553 0.4956 2.018 77.440.07744 0.547 0.729 0.909 0.681 0.5966 1.676 93.22 0.09322 0.651 0.8831.120 0.846 0.7236 1.382 113.06 0.11306 0.779 1.068 1.373 1.041 0.82481.212 128.88 0.12888 0.835 1.202 1.563 1.180 0.8855 1.129 138.36 0.138360.894 1.297 1.689 1.273 0.9870 1.013 154.22 0.15422 0.950 1.427 1.8621.401 1.0900 0.917 170.31 0.17031 1.041 1.574 2.045 1.566 1.1940 0.838186.56 0.18656 1.111 1.706 2.203 1.690 1.3445 0.744 210.08 0.21008 1.1891.892 2.404 1.859 1.4430 0.693 225.47 0.22547 1.212 1.998 2.500 1.9551.5474 0.646 241.78 0.24178 1.258 2.095 2.583 2.042 1.6485 0.607 257.580.25758 1.272 2.180 2.652 2.126

[0225] exposed on HEBS-glass using the number of scan counts as avariable parameter. The data points of Curve A corresponds to 1, 6, 8,10, 12, and 14 scan counts. Curve B displays the data of e-beam exposureusing Cambridge EBMF 10.5 e-beam writer operated at 20 kv, 25 na and 0.1μm addressing grid spacing. Also shown in FIG. 8 for comparison is thenet optical density values at 436 nm resulting from EVC flood gunexposure at 20 kv.

[0226] The effect of retraces as well as the dependence of e-beaminduced optical density on the variable write parameters of Table 2 areexplained in the section “Heat effect of the Write e-beam” in light of apostulated mechanism of e-beam darkening.

[0227]FIG. 9 exhibits the transmittance spectra of the base glass plate0.090 inch thick of the exemplary glass composition No. 3. The cut offin transmittance i.e., the absorption edge of the base glass is due toelectronic transitions of the constituting chemical elements of the baseglass. As the concentration of the doping with photo inhibitorsincreases, the absorption edge of the base glass shifts to longerwavelengths; namely %T of the base glass reduces in the spectral rangeof uv, then near uv and then blue light as the doping concentration ofphoto inhibitors increases. The concentration of photo inhibitors in theexemplary base glass compositions of Exhibit A was optimized for use atmercury G-line so that the HEBS-glass is totally inert to actinicradiation having wavelengths λ equal to or longer than 436 nm, and has a%T value of more than 88%. The %T values of the exemplary HEBS-glass No.3 is shown in FIG. 9 and Table 3. A value of 88% transmittancecorresponds to 96% internal transmission, since reflection less from twoglass surfaces is 8%. The values of %T and the internal transmission ofthe corresponding base glass 0.090 inch thick are more than 90% and morethan 98% respectively for λ≧436 nm.

[0228] A HEBS-glass plate in general consists of two ion-exchangedsurface glass layers, since both surfaces of a base glass plate were ionexchanged during an ion exchange process. To increase the transmittanceof the HEBS-glass plate at λ<436 nm one may grind off one ion-exchangedsurface glass layer and polish the now anhydrous surface to photomaskquality. The transmittance spectra of HEBS-glass No. 3, 0.086″ thickhaving only one ion-exchanged surface is also shown in FIG. 9. TABLE 2The best polynomial fit equation and the slope of the linear portion ofan electron beam darkening sensitivity curve Write parameters of writeschemes Scheme Accelera- Address- No. and tion ing Beam Wave- ElectronBeam Darkening Sensitivity Curve Slope of the Equation voltage grid sizeCurrent length Y = Optical Density linear portion of No. (kV) (micron)(na) (nm) X = Electron Dosage in milli coulomb/cm² the Sensitivity Curve1 30 0.4 250 365 y = 19708x⁶ − 17787x⁵ + 6181.2x⁴ − 1063.8x³ 85.688x² +3.3806x 6.2767 2 30 0.4 250 435 y = −15440x⁶ + 12082x⁵ − 3761.5x⁴ +555.15x³ − 40.414x² + 10.637x 9.2113 3 30 0.4 250 530 y = 46062x⁶ −38146x⁵ + 12229x⁴ − 20139x~ + 173.69x² − 5.8097x 12.507 4 30 0.4 250 630y = 51961x⁶ − 43905x⁵ + 1440_(2x) ⁴ − 2361.2x³ + 197.27x² + 2.2436x9.5929 5 30 0.2 75 365 y = 4788.8x⁶ − 4881.1x⁵ + 1822.8x⁴ − 308.43x³ +19.251x² + 4.098x 4.3024 6 30 0.2 75 435 y = 3780.7x⁶ − 4395.5x⁵ +1959x⁴ − 421.14x³ + 40.268x² . 4.882x 6.1553 7 30 0.2 75 530 y =4227.3x⁶ − 4897.4x⁵ + 2192.6x⁴ − 490.42x³ + 50.025x² + 8.8341x 8.6203 830 0.2 75 630 y = 3750.9x⁶ − 4226.71 + 1854.8x⁴ − 408.5x³ + 41.902x² +4.8131x 3.4022 9 30 0.2 100 365 y − 355.78x⁶ + 406.62x⁵ − 243.17x⁴ +62.851x³ − 12.485x² + 5.5571x 4.424 10 30 0.2 100 435 y = 692.18x⁶ +804.39x⁵ − 358.96x⁴ + 75.143x³ − 10.535x² + 6.9982x 6.1269 11 30 0.2 100530 y = − 175.37x⁶ + 38.823x⁵ + 112.01x⁴ − 80.56x³ + 14.073x² + 7.6887x8.3914 12 30 0.2 100 630 y = − 839.24x⁶ + 947.64x⁵ − 359.07x⁴ + 40.139x³− 0.7855x² + 6.3643 6.4234x 13 30 0.2 125 365 y = − 664.62x⁶ + 932.44x⁵− 464.2x⁴ + 95.04x³ − 13.314x² + 6.4665x 5.392 14 30 0.2 125 435 y =−900.79x⁶ + 1480.9x⁵ − 905.98x⁴ + 243.25x³ − 32.801x² + 7.7152 9.8528x15 30 0.2 125 530 y = 1283.3x⁶ + 1929.4x⁵ 1020x⁴ + 210.47x³ − 22.431x² +12109x 10.672 16 30 0.2 125 630 y = 111.03x⁶ − 345.7x⁵ + 352.69x⁴ −158.9x³ + 24.887x² + 6.7982x 8.1056 17 30 0.2 150 365 y = −104.68x⁶ +149.86x⁵ − 51.158x⁴ − 8.925x³ − 0.3208x² + 5.866x 5.4107 18 30 0.2 150435 y = −341.18x⁶ + 643.91x⁵ − 430.4x⁴ + 115.13x³ − 16.314x² + 9.1502x7.8427 19 30 0.2 150 530 y = −237.51x⁶ + 304.15x⁵ − 52.05x⁴ − 65.071x³ +15.235x² + 10.164x 11.048 20 30 0.2 150 630 y = 225.42x⁶ − 507.45x⁵ +442.41x⁴ − 182.81x³ + 27.588x² + 6.9154x 3.8774 30 20 0.2 100 365 y =1165x⁶ − 1729.1x⁵ + 969.72x⁴ − 255.38x³ + 28215x² + 1.9949x 3.1581 22 200.2 100 435 y = 321.26x⁶ − 495.79x⁵ + 299.53x⁴ − 93.047x³ + 11.878x² +3.99x 4.4463 23 20 0.2 100 530 y = 530.82x⁶ − 893.24x⁵ + 604.38x⁴ −205.16x³ − 28.195x² + 4.3652x 5.7739 24 20 0.2 100 830 y = 747.21x⁶ −1197.4x⁵ + 741.5x⁴ − 217.47x³ + 24.784x² + 2.8845x 3.8774 25 20 0.2 125365 y = −454.78x⁶ + 748.41x⁵ − 467.28x⁴ + 137.8x³ − 22.463x² + 4.8643x3.0043 26 20 0.2 125 435 y = −399.43x⁶ + 659.66x⁵ − 409.6x⁴ + 113.52x³ −15.916x² + 5.6722x 4.5474 27 20 0.2 125 530 y = −46.317x⁶ − 22.298x⁵ +112.29x⁴ − 77.504x³ + 13.876x² + 5.0038x 5.7824 28 20 0.2 125 630 y =417.51x⁶ − 711.04x⁵ + 469.27x⁴ − 145.5x³ + 16.454x² + 3.218x 3.8484 2920 0.2 150 365 y = −74.993x⁶ + 11 8.24x⁶ − 57.1 74x⁴ + 5.249.2x³ −0.6172x² + 3.2267 3.3699x 30 20 0.2 150 435 y = −278.14x⁶ + 503.66x⁵ −329.14x⁴ + 89.552x³ − 11.422x² + 5.4742x 4.7421 31 20 0.2 150 530 y =3.461x⁶ − 102.55x⁵ + 172.08x⁴ − 102.48x⁴ + 18.951x² + 4.8104x 5.872 3220 0.2 150 630 y = 161.84x⁶ − 348.51x⁵ + 286.3x⁴ − 107.41x³ + 13.817x² +3.2921x 3.8027 33 20 0.2 175 385 y = 7.7262x⁶ − 37.019x⁵ + 56.546x⁴ −35.995x³ + 6.9532x² + 2.7738x 3.1017 34 20 0.2 175 435 y = 45.959x⁶ −160.56xx⁵ − 213.4x⁴ − 135.08x³ + 37.556x² + 0.3871x 4.6256 35 20 0.2 175530 y = 82.35x⁶ − 237.03x⁵ + 259.77x⁴ − 130.11x³ + 23.171x² + 4.6292x6.0434 38 20 0.2 175 830 y = 138.78x⁶ − 316.52x⁵ + 272.92x⁴ − 106.53x³ +14.299x² + 3.2743x 3.7417 37 20 0.1 25 385 y = − 257.78x⁶ + 493.22x⁵ −357.35x⁴ + 125.26x³ − 24.07x² + 4.2726x 2.454 38 20 0.1 25 435 y = −421.26x⁶ + 736.88x⁵ − 503.36x⁴ + 168.42x³ − 30.486x² + 3.5813 5.7851x 3920 0.1 25 530 y = 55.321x⁶ − 40.121x⁵ − 7.75x⁴ + 13.643x³ − 8.1316x² +5.4291x 4.7759 40 20 0.1 25 630 y = −199.22x⁶ + 369.63x⁵ − 264.28x⁴ +94.119x³ − 20.554x² + 4.7848x 2.9976

[0229] The internal transmittance of one ion exchanged surface glasslayer was measured by placing the HEBS-glass plate 0.086″ thick with oneion exchanged surface in the sample beam of the U2000 spectrophotometerand placing a base glass plate 0.090 inch thick in the reference beam.The internal transmittance from 350 nm to 500 nm of the ion exchangedglass layer of the exemplary HEBS-glass No. 3 is listed in Table 3.

[0230] Also listed in Table 3 are the corresponding transmittance valuesof the base glass plate 0.090 inch thick, the HEBS-glass plate 0.090inch thick having two ion exchanged surface glass layers (i.e. 2 IEedsurfaces) and the HEBS-glass plate 0.086″ thick having one ion exchangedsurface glass layers (i.e. 1 IEed surface).

[0231] Accelerated test on stability of HEBS-glass No. 3 under intenseactinic exposure at 436 nm was carried out. HEBS-glass No. 3, 0.090 inchthick, having a transmittance value of 89.2% was exposed for a durationof 30 days to 586 milli watt/cm² light intensity at 436 nm from theoutput actinic radiation of a 200 watt mercury arc lamp, the actinicradiation being filtered with an interference filter and focused to aspot of 5 mm diameter. The transmittance value remains constant at 89.2%before and after the intense G-line exposure for 30 days.

[0232] From accelerated tests using focused 365 nm radiation from the200 watt mercury arc lamp, it has been determined that the residualsensitivity of the exemplary HEBS-glass No. 3 to I-line at 365 nm is notdetectable for optical lithographic exposure of less than about onemillion I-line stepper exposures.

[0233] The grayscale mask made of HEBS-glass No. 3 can in general beemployed in I-line as well as G-line optical lithographic exposuresystems.

[0234] An exchange of H⁺ and/or H₃O⁺ ions for alkali metal ions takesplace concurrently with the exchange of Ag⁺ ions for alkli-metal ionswhen HEBS-glass is ion exchanged in an acidic aqueous solutioncontaining silver ions. As a result, H⁺ and/or H₃O⁺ ions entered intothe silicate glass network and silanol groups SiOH formed in the glassnetwork. The formation of the silanol groups in a silicate glass networkis referred to as hydration of glass. HEBS-glass was hydrated, and amoving boundary type concentration profile formed. When water speciesare among the diffusion species in glass, the diffusion of water species(i.e., H⁺ and/or H₃O⁺) and Ag⁺ ions through a hydrated layer isaccompanied by an instantaneous and irreversible immobilization of thediffusion species at the boundary surface. The moving boundary typediffusion profile is due to the fact that the diffusion coefficient ofH⁺, H₃O⁺ and Ag⁺ in the hydrated layer is many order of magnitude largerthan that in the anhydrous base glass.

[0235] An essential feature of diffusion accompanied by an instantaneousand irreversible immobilization of the diffusion species is that a sharpboundary surface moves through the medium, separating a region in whichall of the sites are occupied from one in which none are occupied. Infront of the advancing boundary the concentration of freely diffusingspecies is zero whereas behind it immobilization is complete.

[0236]FIG. 1 is a qualitative representation of the result of silver ionexchange of HEBS-glass in an acidic aqueous solution containing solubleionic silver. There exists a leached surface glass layer, x₁ inthickness, wherein essentially all of the alkali ions are leached outinstead of being exchanged by Ag⁺ ions. The leached surface glass layeris essentially fused silica in composition and contains little or nomobile ions such as sodium, potassium and lithium ions. The exemplaryHEBS-glass No. 3 have a leached surface glass layer of less than about0.5 μm, i.e., x₁<0.5 μm and has an e-beam sensitized glass layer of 3μm, i.e., x₂−x₁=3 μm. HEBS-glass photomask blanks having an e-beamsensitized glass layer (x₂−x₁) of 2 μm, 4 μm, 5 μm and other thickness'were fabricated by controlling the ion exchange duration and/or heatschedules.

[0237] By controlling the operation parameters of the solution ionexchange reactions, the thickness of the e-beam sensitized glass layercan be controlled precisely.

Heat Effect of the Write E-Beam

[0238] The net optical density is a function of the e-beam exposurescheme and the e-beam write parameters. This is because the e-beamsensitivity of HEBS-glass is enhanced by the heating effect of the writebeam. Listed in Exhibit B is the input-power density from e-beamexposure for the three exposure schemes of FIG. 8, where input-powerdensity is equal to (beam current)×(beam acceleration voltage)/(beamspot size).

[0239] The rate of temperature increase at the beam exposure spot i.e.the e-beam exposed volume of HEBS-glass, is proportional to the netpower density which is defined herein as the rate of input-power densityminus the rate of heat dissipation. The rate of heat dissipation islarger for a smaller exposed volume corresponding to a smaller beam spotsize. This is because the ratio of surface area to mass is larger for asmaller volume. The rate of heat dissipation relative to the rate ofinput-power density, together with the corresponding beam spot size arealso listed in Exhibit B.

[0240] Comparing the write parameters that we employed using MEBES IIIwith those of Cambridge EBMF 10.5. It is apparent from Exhibit B thatthe net power density of the MEBES exposure scheme is a factor of 10 to100 times that of the Cambridge exposure scheme (20 kV, 0.1 μm address,0.25 μm spot size, 25 na). The very big difference in heat accumulationbetween the above described exposure parameters of MEBES m and CambridgeEBMF 10.5 should contribute to the observed difference in the e-beaminduced optical density.

[0241] The following hypothesis explains the reason why the e-beamsensitivity of HEBS-glass is enhanced by the heat effect of the writebeam. During e-beam exposure, silver specks of atomic dimensions wereformed from silver halide-alkali halide complex crystals. The formationof a silver speck consisting of 2, 3, or more atoms requires thedeformation of silver halide lattice to silver lattice. Cycles oflattice vibration of sufficient amplitude is necessary to cause theformation of the silver specks. Since larger amplitudes of latticevibrational modes exist at higher temperatures, silver specks are formedmore quickly at a higher temperature. Each retrace (i.e. each scancount) allows an extended time period for the formation of the silverspecks.

[0242] The e-beam sensitivity of HEBS-glass using the exposure scheme ofthe EVC flood exposure system is further elaborated below. Although theinput-power density of EVC exposure scheme is very little compared tothat of Cambridge EBMF 10.5, the rate of heat dissipation during EVCexposure is very small relative to the input-power density, due to theenormous beam exposure spot size of 8 inches Therefore, a large fractionof the input-power density is accumulated throughout the TABLE 3Transmittance Values (% T) of HEBS-glass No. 3 Internal baseTransmittance glass HEBS-glass HEBS-glass of an IEed Wavelength 0.090″ 2IEed Surface 1 IEed Surface glass layer (nm) (% T) (% T) (% T) (% T)500.0 91.2 90.6 90.5 100 495.0 91.0 90.5 90.2 100 490.0 91.0 90.6 90.2100 485.0 91.0 90.4 90.3 100 480.0 90.9 90.2 90.0 100 475.0 90.8 90.490.0 100 470.0 90.7 89.9 89.9 100 465.0 90.7 89.9 89.7 100 460.0 90.689.9 89.7 100 455.0 90.6 89.6 89.7 100 450.0 90.5 89.6 89.4 100 445.090.4 89.4 89.4 100 440.0 90.4 89.2 89.3 100 435.0 90.4 89.2 89.1 99.9430.0 90.2 88.6 89.0 99.9 425.0 90.2 88.6 88.8 99.6 420.0 90.1 88.0 88.799.6 415.0 90.0 87.6 88.3 99.3 410.0 89.9 86.9 88.0 99.0 405.0 89.9 86.187.6 98.7 400.0 89.9 84.9 87.0 98.1 395.0 89.7 83.3 86.3 97.4 390.0 89.781.3 85.3 96.3 385.0 89.5 78.7 83.9 94.9 380.0 89.4 75.0 82.1 92.7 375.089.1 71.2 79.9 90.5 370.0 88.4 65.7 76.8 87.6 365.0 87.1 59.1 72.3 83.7360.0 84.7 51.2 66.7 79.1 355.0 80.0 41.9 58.8 73.6 350.0 71.6 31.3 48.266.9

[0243] flood exposure duration. Moreover, HEBS-glass was under EVC floodgun exposure for a long duration of e.g. 10 minutes which corresponds to600×10¹² periods of lattice vibration. There is thus plenty of time forthe deformation of silver halide lattice into silver lattice.

[0244] For the choice of write parameters using any e-beam writer towrite a gray level mask it is helpful to consider the HEBS-glassproperties which are summarized immediately below.

[0245] HEBS-glass is more sensitive using a larger beam current densityand a larger beam spot size, since the sensitivity is enhanced by theheating effect of the write beam. At a given current density, a largerspot size is beneficial since heat dissipation is slower. Using a highkV beam and a spot size of up to about 0.2 μm, the resolution of therecorded image in HEBS-glass is primarily determined by the scatteringof the electrons in glass. To a certain extent, one can increase boththe spot size and the beam current to maximize the throughput withoutthe adverse effect of reducing resolution.

[0246] A much larger beam current density than that normally used inexposing photoresist can be employed and is recommended for e-beam writeon HEBS-glass.

[0247] Using a vector scan e-beam writer, a large range of availableclock rates is the key factor to produce a large optical density rangeof gray levels. The maximum clock rate of an e-beam writer is employedto produce the minimum optical density level of the gray levels. Thelarger the maximum clock rate the higher the currant density that can beemployed and therefore higher throughput.

[0248] When a vector scan e-beam writer has a limited range of usefulclock rates, one retrace (i.e. two scan counts) can be utilized todouble the optical density values for all phase levels provided thelinear region of the sensitivity curve is utilized.

[0249] Number of retraces can be employed as a variable parameter usinga raster scan e-beam writer. For example, 16 phase levels can beobtained using 1 to 16 scan counts, namely, using 1 scan count to exposethe lowest optical density level and using 16 scan counts to write thehighest optical density level.

[0250] Although the e-beam sensitivity of HEBS-glass is a function ofthe exposure scheme and write parameters, e-beam induced optical densityin the HEBS-glass is a unique function of electron dosage for a givenset of write parameters. Therefore, the net optical density versuselectron dosage is very reproducible. Exhibit B Input-power density andrate of heat dissipation. Input-Power Beam Relative Rate of Density(Watt/cm²) Spot Size Heat Dissipation MEBES III 8 × 10⁶ 1 μm Small 1scancount Cambridge 8 × 10⁵ 0.25 μm Large EBMF 10.5 EVC 0.1 8 inch Verysmall

Coloring Speck of Silver

[0251] Upon e-beam exposure coloring specks of silver are formed in theHEBS-glass. Since there are no chemical or physical development steps,the silver specks are of atomic dimensions and the image has nograininess. The recorded image has a continuous tone even when observedat the highest magnification under microscope e.g. 1500×. On thecontrary, at this high magnification the image in a conventional highresolution photographic emulsion plate is intrinsically halftone,because isolated grains of photographic emulsion plates resemblingdispersed halftone dots exist at gray levels of low optical densityvalues.

Sub 0.25 Micrometer Resolution

[0252] Since there is no graininess, HEBS-glass is capable of very highresolution. Sub 0.25 μm features were written in the exemplaryHEBS-glasses of Exhibit A .

[0253] Vertical resist profile exists at the boundary of sub micronresist features which were printed using HEBS-glass masks.

HEBS-Glass Gray Scale Masks with Multi-Gray Levels

[0254]FIG. 10 exhibits an optical micrograph of a gray scale mask whichis a grating in HEBS-glass No.3 The grating has 250 gray levels within aperiod of 200 micrometers. A Cambridge EBMF 10.5 e-beam writer operatedat 20 kV, 0.2 μm addressing grid size and 100 na beam current, wasemployed to fabricate the grating with 250 predetermined optical densityvalues. The maximum and minimum optical density value are 1.6 and 0.149respectively. The gray levels have an equal interval of transmittance,namely the values of the transmittance T corresponding to 250 graylevels are

Ti=0.709578−(0.684459/249)i

[0255] Where i=0, 1, 2, - - - to 249

[0256] The gradual and smooth increase in the transmittance within eachgrating period seen in FIG. 10 should result in analog resist profilehaving smooth continuous blaze surfaces.

[0257]FIG. 11 exhibits an optical micrograph of a portion of a grayscale mask made of HEBS-glass No. 3. The primary mask pattern is adiffractive optical lens having ten gray levels in each zone. The widthof 5 complete zones represented in the photo are 920 μm, 500 μm, 390 μm,330 μm and 290 μm. Ten gray levels in the range of 0 to 1.0 unit ofoptical density values, were written in HEBS-glass No. 3 using CambridgeEBMF 10.5 e-beam writer operated at 20 kv acceleration voltage, 0.2 μmaddressing grid size and 150 na beam current. It is seen in FIG. 11, asthe zone width reduces from 920 μm the discreet gray levels become lessapparent.

[0258] A second pattern which consists of parallel lines with a 42 μmpitch was written within the lens pattern to demonstrate that apredetermined optical density value can be added to the existing maskpattern in HEBS-glass. Since the parallel lines was written with aconstant e-beam dosage, the optical density along each lines increasesas it enters into a darker gray level of the lens pattern.

[0259] One of the products of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0260] (a) the transmittance is more than 88% at 436 mn

[0261] (b) upon exposure to an electron beam with an electron beampattern generator operated at a value of acceleration voltage selectedfrom 20 to 30 kV, a value of addressing grid size selected from 0.1 to0.4 micrometer, and a value of beam current selected from 25 to 250 na,the electron beam darkening sensitivity in the linear portion of thesensitivity curve, is at least 2.454 unit of optical density value inthe spectral range of 365 nm to 630 nm per milli coulomb/cm².

[0262] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0263] Another of the product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0264] (a) the transmittance is more than 88% at 436 nm

[0265] (b) upon exposure to an electron beam using an electron beampattern generator operated with a write scheme having a value ofacceleration voltage selected from 20 to 30 kV, a value of addressinggrid size selected from 0.1 to 0.4 micrometer, and a value of beamcurrent selected from 25 to 250 na, the electron beam darkeningsensitivity in the linear portion of the sensitivity curve, is at least2.454 unit of optical density value in the spectral range of 365 nm to630 nm per milli coulomb/cm². The write scheme is selected from thewrite schemes of Table 2, said electron beam darkening sensitivity ofthe HEBS-glass is substantially represented by the sensitivity curvecorresponding to that of the chosen write scheme of Table 2.

[0266] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0267] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0268] (a) the transmittance is more than 88% at 436 nm

[0269] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.4 micrometer and a beamcurrent of 250 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0270] Y=19708x⁶−17787x⁵+6181.2x⁴−1063.8x³+85.688x²+3.3806x

[0271] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0272] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0273] (a) the transmittance is more than 88% at 436 nm

[0274] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.4 micrometer and a beamcurrent of 250 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0275] Y=−15440x⁶+12082x⁵−3761.5x⁴+555.15x³−40.414x²+10.63x

[0276] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO_(2.)

[0277] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0278] (a) the transmittance is more than 88% at 436 nm

[0279] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.4 micrometer and a beamcurrent of 250 na will darken to a net optical density value Y at 530 nmsubstantially in accordance with the equation stated immediately below;

[0280] Y=46062x⁶−38146x⁵+12229x⁴−2013.9x³+173.69x²+5.8097x

[0281] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0282] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0283] (a) the transmittance is more than 88% at 436 nm

[0284] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.4 micrometer and a beamcurrent of 250 na will darken to a net optical density value Y at 630 nmsubstantially in accordance with the equation stated immediately below;

[0285] Y=51961x⁶−43905x⁵+14402x⁴−2361.2x³+197.27x²+2.2436x

[0286] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0287] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0288] (a) the transmittance is more than 88% at 436 nm

[0289] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV. an addressing grid size of 0.1 micrometer and a beamcurrent of 25 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0290] Y=−257.78x⁶+493,22x⁵357.35x⁴+125.26x³−24.07x²+4.2726x

[0291] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0292] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0293] (a) the transmittance is more than 88% at 436 nm

[0294] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.1 micrometer and a beamcurrent of 25 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0295] Y=421.26x⁶+736.88x⁵−503.36x⁴+168.42x³−30.486x²+5.785x

[0296] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0297] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0298] (a) the transmittance is more than 88% at 436 nm

[0299] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.1 micrometer and a beamcurrent of 25 na will darken to a net optical density value Y at 530 nmsubstantially in accordance with the equation stated immediately below;

[0300] Y=55.321x⁶−40.121x⁵−7.75x⁴+13.643x³−8.13 16x²+5.4291x

[0301] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0302] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0303] (a) the transmittance is more than 88% at 436 nm

[0304] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.1 micrometer and a beamcurrent of 25 na will darken to a net optical density value Y at 630 nmsubstantially in accordance with the equation stated immediately below;

[0305] Y=−199.22x⁶+369.63x⁵−264.28x⁴+94.119x³−20.554x²+4.7848x

[0306] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 and 58.2 to 78.8% SiO₂.

[0307] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0308] (a) the transmittance is more than 88% at 436 nm

[0309] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 75 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0310] Y=4788.8x⁶−4881.1x⁵+1822.8x⁴−308.43x³+19.251x²+4.098x

[0311] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0312] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0313] (a) the transmittance is more than 88% at 436 nm

[0314] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 75 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0315] Y=3780.7x⁶−4395.5x⁵+1959.1x⁴−421.14x³+40.268x²+4.882x

[0316] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0317] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0318] (a) the transmittance is more than 88% at 436 nm

[0319] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 100 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0320] Y=−355.78x⁶+466.62x⁵−243.17x⁴+62.85 1x³−12.485x²+5.5571x

[0321] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0322] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0323] (a) the transmittance is more than 88% at 436 nm

[0324] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 100 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0325] Y=−692.18x⁶+804.39x⁵−358.96x⁴+75.143x³−10.535x²+6.9982x

[0326] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0327] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0328] (a) the transmittance is more than 88% at 436 nm

[0329] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 125 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0330] Y=−664.62x⁶+932.44x⁵−464.2x⁴+95.04x³−13.3 14x²+6.4665x

[0331] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0332] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0333] (a) the transmittance is more than 88% at 436 nm

[0334] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 125 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0335] Y=−900.79x⁶+1480.9x⁵−905.98x⁴+243.25x³−32.801x²+9.8528x

[0336] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0337] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0338] (a) the transmittance is more than 88% at 436 nm

[0339] (b) upon exposure to a value, X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 150 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0340] Y=−104.68x⁶+149.86x⁵51.158x⁴−8.925x³−0.3208x²+5.866x

[0341] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0342] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0343] (a) the transmittance is more than 88% at 436 nm

[0344] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 30 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 150 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0345] Y=−341.18x⁶+643.91x⁵−430.4x⁴+115.13x³−16.314x²+9.1502x

[0346] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0347] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0348] (a) the transmittance is more than 88% at 436 nm

[0349] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 100 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0350] Y=1165x⁶1729.1x⁵+969.72x⁴−255.38x³+28.215x²+1.9949x

[0351] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0352] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0353] (a) the transmittance is more than 88% at 436 nm

[0354] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 100 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below,

[0355] Y=321.26x⁶−495.79x⁵+299.53x⁴−93.047x³+11.878x²+3.99x

[0356] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0357] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0358] (a) the transmittance is more than 88% at 436 nm

[0359] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 125 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0360] Y=−454.78x⁶+748.41x⁵−467.28x⁴+137.8x³−22.463x²+4.864x

[0361] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0362] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0363] (a) the transmittance is more than 88% at 436 nm

[0364] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 125 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0365] Y=−399.43x⁶+659.66x⁵−409.6x⁴+113.52x³−15.916x²+5.6722x

[0366] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0367] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0368] (a) the transmittance is more than 88% at 436 nm

[0369] (b) upon exposure to a value X in milli coulomb/cm of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 150 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below,

[0370] Y=−74.993x⁶+118.24x⁵−57.174x⁴+5.2492x³−0.6172x²+3.3699x

[0371] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0372] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0373] (a) the transmittance is more than 88% at 436 nm

[0374] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 150 na will darken to a net optical density value Y at 435 nmsubstantially in accordance with the equation stated immediately below;

[0375] Y=−278.14x⁶+503.66x⁵−329.14x⁴+89.552x³−11.422x²+5.4742x

[0376] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0377] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0378] (a) the transmittance is more than 88% at 436 nm

[0379] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 175 na will darken to a net optical density value Y at 365 nmsubstantially in accordance with the equation stated immediately below;

[0380] Y=7.7262x⁶−37.019x⁵+56.546x⁴−35.995x³+6.9532x²+2.7738x

[0381] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0382] Another product of the present invention is a transparentHEBS-glass which in bodies of 0.090 inch cross section will exhibit thefollowing properties:

[0383] (a) the transmittance is more than 88% at 436 nm

[0384] (b) upon exposure to a value X in milli coulomb/cm² of electrondosage with an electron beam writer operated at a beam accelerationvoltage of 20 kV, an addressing grid size of 0.2 micrometer and a beamcurrent of 175 na will darken to a net optical density value Y at 530 nmsubstantially in accordance with the equation stated immediately below,

[0385] Y=82.35x⁶−237.03x⁵+259.77x⁴−130.11x³+23.171x²+4.6292x

[0386] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂.

[0387] The products of the present invention described above haveutility in making a gray scale mask with multi-gray levels, each of saidgray levels having a predetermined optical density value which isobtained through exposure to a predetermined electron dosage, said grayscale mask can be utilized in making three divisional microstructureswith general three dimensional surfaces in photoresist through a singleoptical exposure in a photolithographic process.

[0388] The profile of a three dimensional surface is transferred into asubstrate material using an etching process.

[0389] For example, one of the products of the present invention is atransparent HEBS-glass which in bodies of 0.090 inch cross section willexhibit the following properties:

[0390] (a) the transmittance is more than 88% at 436 nm

[0391] (b) upon exposure to an electron beam with an electron beampattern generator operated at a value of acceleration voltage selectedfrom 20 to 30 kV, a value of addressing grid size selected from 0.1 to0.4 micrometer, and a value of beam current selected from 25 to 250 na,the electron beam darkening sensitivity in the linear portion of thesensitivity curve, is at least 2.454 unit of optical density value inthe spectral range of 365 nm to 630 nm per milli coulomb/cm².

[0392] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali: metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂ has utility in making a gray scale mask with multi-gray levels,each of said gray levels having a predetermined optical density valuewhich is obtained through exposure to a predetermined electron dosage,said gray scale mask can be utilized in making three dimensionalmicrostructures with general three dimensional surfaces in photoresistthrough a single optical exposure in a photolithographic process.

Cost Efficient Mass Fabrication of Diffractive Optical Elements (DOEs)

[0393] HEBS-glass is a mask material sensitive towards e-beam exposure,and exposure with a certain electron beam dosage changes the opticaldensity of the material. After e-beam exposure the mask needs no furtherdevelopment or fixation process. The mask with multi levels of opticaldensities can then be used to expose a photo resist in a contact aligneror in a reduction stepper. This allows to associate a certain resistthickness after development with each optical density. The informationwas used to determine the e-beam dosages for each of the (e.g. 32) phaselevels necessary to generate a Diffractive Optical Elements (DOE hereinafter). The so generated HEBS-glass gray level mask can be used toexpose numerous DOEs using an optical lithography tool. After manycopies of the mask on the photo resist are developed, many substrateswith the developed photo resist are placed in a Chemically Assisted IonBeam Etching (CAIBE) system, to simultaneously transfer themicrostructures from the analog resists onto the surfaces of thesubstrates. An overview of these processing steps is shown in FIG. 12.

[0394] The fabrication of DOE arrays using a HEBS-glass gray scale maskwith multi gray levels and the process steps of FIG. 12 was described in“cost-effective mass fabrication of multi-level diffractive opticalelements by use of a single optical exposure with a gray scale mask onhigh-energy beam sensitive glass” Applied Optics, Jul. 10, 1997/Vol. 36,No. 20 by Walter Daschner, Pin Long, Robert Stein, Che-Kuang (Chuck) Wuand S. H. Lee.

[0395] The described fabrication method shows the cost effective massfabrication of DOEs. There are a number of advantages:

[0396] The mass fabrication is simplified and more cost-effective.Instead of a set of masks (i.e. 5 masks for 32 phase levels) with allthe associated resist processing, only a single mask needs to be exposedin the e-beam writer and no resist processing is associated with themask generation. Phase levels of DOE are immediately visible as graylevels in HEBS-glass upon e-beam exposure.

[0397] All phase levels are written in a single step on a single mask.The inevitable mis-registrations and associated efficiency lossesbetween subsequent exposures are avoided.

[0398] Third, the number of processing steps for the DOE fabricationcompared to binary mask fabrication of e.g. 32 phase levels is reducedby a factor of 5. This will reduce the cost for high quality monolithicDOEs substantially.

[0399] Fourth, even with a binary fabrication method for masterfabrication and a following replication step based on injection moldingthis replication method only becomes economic with a number of DOEs tobe fabricated in the 10's of thousands. Since the proposed fabricationmethod greatly reduces the involved fabrication steps resulting in acost reduction, the number at which molding based methods becomeeconomically feasible will grow considerably. This will allow to avoidthe problems associated with the molding approach. The material which isbest suited for the application, can be chosen without being limited bythe constraints of the molding material (i.e. limited temperature rangeof operation or limited wavelength range). Also all the involvedmaterials and tools are compatible with VLSI fabrication so that no newfabrication or software tools need to be established unlike in the caseof replication by injection molding or casting.

[0400] There is a considerable gain in turn around time since the numberof production steps has been reduced and the mask fabrication steps havebeen simplified.

Cost Efficient Mass Fabrication of Asymmetric Irregularly ShapedMicro-Lens Arrays

[0401] A cost-effective way of fabricating large arrays of refractivemicro lenses becomes more and more important. Gray level maskfabrication offers the possibility to shape arbitrary resist profilesand therefore produce arrays of general aspheric non rotationallysymmetric refractive lenses with different functionality, completeaberration correction and a 100% fill-factor. The fabrication methodbased on HEBS-glass gray level mask allows for complete freedom in termsof the shapes e.g. asymmetric, irregularly shaped lenses, and locationof the lenses e.g. with accurate center to center spacing.

[0402] As the resist for the lithography step a comparatively thickphotoresist can be employed in order to achieve a resist thickness inthe range of up to 30 microns. This feature depth in resist will allowfor a total lens sag after the etching transfer of up to about 120microns, since a magnification of the feature depth of about a factor of3 to 5 can be achieved during the transferring step of resist profilesinto their respective substrates via Chemically Assisted Ion BeamEtching (CAIBE). The described fabrication steps are shown in FIG. 13.

[0403] For the analog transfer scheme of FIG. 13, i.e. from an opticaldensity profile in the gray-level mask into a surface height profile inthe photoresist, it is necessary that the number of gray levels beincreased as the aperture of the refractive lens increases; for example,HEBS-glass masks having a minimum of 32, 64, and 96 gray levels aredesirable to fabricate refractive micro lenses having apertures of 50μm, 100 μm, and 200 μm, respectively.

[0404] The fabrication of refractive microlens arrays using a HEBS-glassgray level mask and the processing steps of FIG. 13 was described in“General aspheric refractive micro-optics fabricated by opticallithography using a high energy beam sensitive glass gray-level mask” J.Vac. Sci. Technol. B 14 (6), November/December 1996 by Walter Daschner,Pin Long, Robert Stein, Che-kuang (Chuck) Wu and S. H. Lee.

The Analog Transfer Scheme Using HEBS-glass Gray Scale Masks

[0405] It has been determined that the procedures of processingphotoresist to produce analog resist profile can be derived from thosenormally used for binary photo lithography with the followingmodifications and provisions:

[0406] 1. positive and non chemically amplified Novalac basedphotoresist is preferred

[0407] 2. prebake photoresist at a low temperature and for a shortenedtime duration from that is normally used for binary lithography

[0408] 3. use a weak developer, or dilute the usual developers forexample, by a factor of 4.

Exemplary Calibration Curves

[0409] (a) Surface height versus optical density, i.e. the calibrationcurve of the analog transfer scheme

[0410]FIG. 14 shows the remaining thickness after development of ShipleyS1650 photoresist as a function of the optical density at 436 nm of thegray levels in a HEBS-glass mask. The initial (i.e. as coated) thicknessof the Shipley S1650 photoresist was 4.0 μm. The range of resistthickness in the depth versus optical density calibration curve can bealtered through the choice of a photoresist and/or resist parameters,the initial thickness of the photoresist in particular. In the plots ofresist thickness versus optical density, the slope of the calibrationcurve reduces as the developed resists thickness approaches the initial(as coated) resists thickness. Therefore, to produce an analog resistprofile of a given feature depth, it is necessary to start with an ascoated resist thickness which is more than that of its required featuredepth.

[0411] To transfer multilevel resist structure of DOE into quartzthrough a dry etching process, the relative etch rate betweenphotoresist and quart substrate can be so chosen to achieve the finalneeded etch depth 3 to 6 times that in the resist. Therefore, for thefabrication of DOE in quartz, a surface height variation of e.g. 500 nmin the resist results in a depth modulation in quartz of up to 3000 nm.

[0412] (b) Electron beam darkening sensitivity curve (Optical densityversus electron dosage).

[0413] Table 2 list, FIG. 7 and FIG. 8 depict the exemplary e-beamdarkening sensitivity curves of HEBS-glass No.3. For DOE fabrication therequired optical density values of a HEBS-glass gray level mask aretypically in the range of 0.1 to 1.2.

[0414] For the fabrication of refractive micro lens arrays, the opticaldensity levels in a HEBS-glass gray level mask is in general in therange of 0 to more than 1.2. The maximum optical density value is largerto produce a larger lens-sag.

[0415] (c) Optical density versus clock rate, i.e. the calibration curveof e-beam exposure.

[0416] The electron dosage D in micro coulomb/cm² is calculated asfollows:

D(μc/cm²)=I·N=I·N/f

[0417] Where I is bean current in amp., t is exposure duration i.e.dwell time per pixel in μsec, and N is number of pixels in 1 cm². Theexposure duration per pixel is equal to 1/f, where f is the clock ratei.e. the write frequency. Since the clock rate can be varied on the flyusing a vector scan e-beam writer, the calibration curve of e-beamexposure “optical density versus clock rate” is a practical one for avector scan e-beam writer. The calibration curve was determinedexperimentally for each combination of write parameters which includebeam acceleration voltage, beam current and addressing grid size.

[0418] The net optical density values at 435 nm and the correspondingclock rates are plotted in FIG. 15 and FIG. 16 for two write schemeusing the data of Table 1 and Table 2. FIG. 15 exhibits the calibrationcurve “net optical density versus clock rate” for the e-beam writescheme of 30 kv, 250 na beam current and 0.4 μm addressing grid size.FIG. 16 displays the calibration curve “net optical density versus clockrate” for the e-beam write scheme of 30 kv, 75 na beam current and 0.2μm addressing grid size.

Fabrication of HEBS-glass Gray Level Masks

[0419] A HEBS-glass photomask with multi-gray levels is ideally suitedfor fabrication of diffractive optical elements (DOE), refractive microoptics, micro-electromechanical (MEM) devices,micro-opto-electro-mechanical (MOEM) devices and integrated opticalcomponents, and for beam shaping optics.

[0420] A mask for multi phase levels of DOE is made by exposing in ane-beam writer with predetermined dosages according to a calibrationcurve of the analog transfer scheme such as that shown in FIG. 14together with the e-beam darkening sensitivity curve, examples of whichare listed in Table 2.

[0421] To make a HEBS-glass gray level mask using a vector scan e-beamwriter, optical density levels which will achieve evenly spaced multidepth levels over the thickness range of photoresist needed for asubsequent dry etching, are determined from a calibration curve of theanalog transfer scheme e.g. FIG. 14. Each optical density level in themask is then written with a clock rate corresponding to thepredetermined optical density value. The clock rate is determined fromthe calibration curve of e-beam exposure such as that shown in FIG. 15and FIG. 16. The calculation of the clock rate is further elaboratedbelow.

[0422] The clock rates f were calculated from polynomial equations suchas eq. A and eq. B for a large number of the predetermined opticaldensity levels of gray level mask designs. Eq. A and eq. B are the bestpolynomial fit equations of the experimental data.

1/f=0.0692D ⁶−0.4299D ⁵+1.0403D ⁴−1.2009D ³+0.666D ²+0.5339D  eq. A

1/f=0.012D ⁶−0.0862D ⁵+0.304D ⁴−0.5256D ³+0.5491D ²+0.5622D  eq. B

[0423] Plotted the experimental data of Table 1, FIG. 17 exhibits thecalibration curve “1/(clock rate) versus net optical density at 435 nm”for the e-beam write scheme of 30 kv, 250 na beam current and 0.4μmaddressing grid size. Eq. A is the best polynomial fit equation of thedata points of FIG. 17.

[0424]FIG. 18 displays the calibration curve “1/(clock rate) versus netoptical density at 435 nm” for the e-beam write scheme of 30 kv, 75 nabeam current and 0.2 μm addressing grid size. Eq. B is the best fitpolynomial equation of the data points of FIG. 18.

[0425] HEBS-glass masks having gray levels ranging from just a few tomany hundreds were fabricated via e-beam direct write in HEBS-glass. Forexample, a very high quality, sinusoidal absorption grating 2 cm×2 cm insize having 625 gray levels within each period of 250 μm±0.2 μm wasfabricated using Cambridge EBMF 10.5 e-beam writer. The grating is aseries of linear strips, 2 cm long whose absorbance varies sinusoidallyalong the direction perpendicular to the strips. The linear strip whichis the lines of constant optical density, has a requirement of betterthan ±0.1 μm in linearity. Within each period, the minimum transmissionat the wavelength of 435 nm is 1% of the maximum transmission. 625 clockrates were determined from the eq. A for the grating fabricated usingthe write scheme of Table 1. A different set of 625 clock rates weredetermined from eq. B for a second grating fabricated using a second setof write parameters. The minimum and the maximum optical density valuesof the 625 gray levels are 0.172 and 2.172 using the write scheme ofFIG. 17, and are 0.178 and 2.178 using the write parameters of FIG. 18.

[0426] Using a gray level mask in an optical exposure system, thethroughput of resist exposure in DOE fabrication increases with a lowervalue of the minimum optical density level in the gray level mask. It istherefore desirable to have the optical density value of the lowest graylevel being about or below 0.1.

[0427] A vector scan e-beam writer having the capability of higher clockrates can be employed to increase the throughput of mask making and alsoto reduce the minimum optical density value toward zero. The capabilityof focusing a larger beam current to a given e-beam spot size is animportant feature to take full advantage of a larger clock rate.

[0428] It has been determined that the sensitivity of HEBS-glass isenhanced by the heat effect of a larger beam current. The throughput ofwriting a HEBS-glass gray level mask increases by a factor of 7.5instead of 4 when the addressing grid size is increased from 0.2 μm to0.4 μm, and at the same time the beam current is increased from 75 na to300 na.

[0429] A grounding layer in the form of a 10 nm (or thicker) chromelayer should be applied to the HEBS-glass photomask blank. The onlypurpose of this thermally evaporated layer is to avoid local charging ofthe mask plate during the e-beam writing process. The mask is thenexposed in the e-beam writer.

[0430] After the e-beam exposure the only necessary processing step isthe wet etching of the Cr grounding layer to make the mask ready forexposure in an optical lithography exposure tool. Besides the removal ofthe grounding layer no processing of the mask is necessary.

Microscope Observation

[0431] Phase levels of DOE are immediately visible as gray levels inHEBS-glass upon e-beam exposure. The pattern data or image should beobserved in a transmission mode. Since the pattern data or image inHEBS-glass do not scatter or reflect light, they are essentially notvisible in a reflection mode.

DOE Made of An E-Beam Direct-Write HEBS-Glass Plate Or Made of A LaserBeam Direct-Write LDW-Glass Plate

[0432] The multi-gray levels in HEBS-glass were transformed intomulti-phase levels, i.e., depth variation of surface relief inHEBS-glass upon a wet chemical etching or a dry etching process. Anexemplary etching process consists of dipping an e-beam patternedHEBS-glass plate in 1.25% HF solution for 40 minutes. Before the etchingprocess the image in HEBS-glass causes an absorption or amplitudemodulation of an incoming light beam, whereas after the etching processthe image in HEBS-glass effects a phase modulation of the incoming lightbeam.

[0433] Under microscope observation, the e-beam written pattern isessentially not visible in a reflection mode. After the selectiveetching process the pattern becomes visible in the reflection mode.

[0434] HEBS-glasses having been uniformly darkened with a high energybeam, electron beam in particular, is a laser beam direct write glass,LDW glass herein after.

[0435] The laser beam direct write LDW-glass has a much superiorselective etch ratio than the e-beam direct write HEBS-glass. Due to avery large etch ratio of the laser exposed area vs. unexposed areawithin a LDW-glass mask, DOE as well as refractive micro lens arrays andgeneral three dimensional surfaces can be fabricated in LDW-glass withthe laser direct write approach to result in a LDW-glass mask andfollowed by an etching step.

LDW-Glass Mask Fabrication

[0436] One of the products of the present invention is an LDW-glasswhich is a HEBS-glass having been uniformly darkened to a predeterminedoptical density value. Said predetermined optical density value is atleast the maximum optical density value of a pre-designed gray scalemask with multi-gray levels, said LDW-glass prior to being darkened withan electron beam or a flood electron gun is a transparent HEBS-glasswhich in bodies of 0.090 inch cross section will exhibit the followingproperties:

[0437] (a) the transmittance is more than 88% at 436 nm

[0438] (b) upon exposure to an electron beam with a flood electron gunor with an electron beam pattern generator operated at a value ofacceleration voltage selected from 20 to 30 kV, the electron beamdarkening sensitivity in the linear portion of the sensitivity curve, isat least 2.454 unit of optical density value in the spectral range of365 nm to 630 nm per milli coulomb/cm².

[0439] Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO2, said gray scale mask is made by exposure to a focused laser beam,said multi-gray levels are made using laser write speed and/or laserbeam intensity and/or number of retraces as variable write parameters.

[0440] Another products of the present invention is an LDW-glass whichis a HEBS-glass having been uniformly darkened to a predeterminedoptical density value, said predetermined optical density value is atleast the maximum optical density value of a pre-designed gray scalemask with multi-gray levels, said LDW-glass prior to being darkened withan electron beam or a flood electron gun is a transparent HEBS-glasswhich in bodies of 0.090 inch cross section will exhibit the followingproperties:

[0441] (a) the transmittance is more than 88% at 436 nm

[0442] (b) upon exposure to an electron beam with a flood electron gunor with an electron beam pattern generator operated at a value ofacceleration voltage selected from 10 to 100 kV, the HEBS-glass isdarkened to a predetermined optical density value which is at least themaximum optical density value of a pre-designed gray scale mask withmulti-gray levels.

[0443] IS Said HEBS-glass having a base glass composition consistingessentially on the mole % oxide basis 11.4 to 17.5% of one or morealkali metal oxide, 2.4 to 10.2% total of photosensitivity inhibitorsand RS suppressing agents including 2.4 to 10.2% TiO₂, 1.1 to 2.4%Al₂O₃, 0 to 4.6% B₂O₃, 3.7 to 13.2% ZnO, 0.5 to 6% Cl and 58.2 to 78.8%SiO₂, said gray scale mask is made by exposure to a focused laser beam,said multi-gray levels are made using laser write speed and/or laserbeam intensity and/or number of retraces as a variable write parameters.

[0444] The LDW-glass gray scale mask can be utilized in making threedimensional microstructures with general three dimensional surfaces inphotoresist through a single optical exposure in a photolithographicprocess.

[0445] The profile of a three dimensional surface in photoresist istransferred into a substrate material using an etching process.

[0446] Laser Direct Write glass (LDW-glass) also offers the advantagesof a one step fabrication of a true gray level mask. The exposure ofthis gray level mask is done in a laser writing tool. This also allowsthe use of the existing software previously written to support maskmaking and direct write on resist approaches for the fabrication ofdiffractive optical elements (DOEs). The so generated gray level maskcan be used in an optical lithographic exposure tool (e.g., a G-line ora I-line wafer stepper, or a contact aligner) to mass fabricate resistprofiles.

[0447] Using the LDW gray level mask fabrication and a following opticallithographic exposure, alignment errors are avoided, since the mask iswritten in a single step using different energy densities of a laserbeam to generate multi-gray levels. This new approach also allows a veryeconomical mask fabrication. Instead of fabricating a set of 5 binarychrome masks with all the involved resist processing and wet etching, asingle writing step without the need for any processing is needed. Thissingle mask then contains all the necessary information previouslycontained in a set of 5 binary masks. Misalignment due to sequentialprinting of 5 binary masks in a set is completely avoided. After the LDWgray level mask is fabricated a series of single exposure in astep-and-repeat system can generate hundreds of DOEs on the same wafer.This wafer can then be processed with a single CAIBE step to transferthe DOE structure of a large number of different elements simultaneouslyinto the substrate. Since the complete DOE structure is transferred intothe substrate there is no need for a resist stripping step after theetching process. After dicing the wafer hundreds of monolithicmultilevel DOEs can be generated by a process which cut the involvedprocessing steps by more than a factor of 5.

[0448] The optical density spectra of exemplary LDW-glasses are shown inFIG. 19 and FIG. 20. FIG. 19 exhibits the optical density spectra ofLDW-HR plates-Type I, -Type II and -Type m. FIG. 20 exhibits the opticaldensity spectra of LDW-IR plates-Type I, -Type II and -Type III, LDW-HRplates-Type I, -Type II and -Type III as well as LDW-IR plates-Type I,-Type II and -Type III are HEBS-glass No. 3 having been darkened withflood electron gun, using the acceleration voltage and electron dosageof the electron beam exposure as variable parameters.

[0449] LDW-glass photomask blanks are monolithic silicate glass plateswith no coating of any kind. LDW-glasses contain a large number densityof coloring specks of silver within 1 m in the thickness dimension intothe glass surface. A focused laser beam of any wavelength in thespectral range of near uv, visible (e.g., 514 nm 632 nm and 647 nm),near infrared (e.g. 820 nm and 1060 mm) and infrared (e.g. 10.6 μm) canbe used to heat erase these coloring specks, causing a portion or all ofthe coloring specks of silver in glass to become colorless silver ions.The transmittance of LDW-glass plates increases with increasingwriting-energy density of a focused laser beam. The required writingenergy density is a function of the wavelength of write beam, writingvelocity, i.e., the speed of laser sweep, the intensity profile of thefocused laser beam and the value of %T at the desired gray level. Forexample, having been exposed to an energy density of 2 joule/cm² a writebeam at the wavelength of 514 nm and a writing velocity of 4 meters/sec,LDW-HR plate-type I becomes totally transparent.

[0450] At any given writing velocity, there exists anerasure-threshold-intensity I_(ETh) below which there is no change inoptical density of LDW-glass plates even with multiple retraces. Using awrite-beam intensity above the erasure-threshold-intensity I_(ETh), theoptical density of LDW-glass plates reduces with each additional retraceand the LDW-glass plates can be erased to a transparent state withmultiple retraces. As the write-beam intensity increases-further abovethe erasure-threshold-intensity I_(ETh), retraces needed to bring aboutthe transparent state decrease in number. LDW-glass plates are madetransparent in one laser sweep i.e., no retraces at afull-erasure-intensity I_(FE).

[0451] At any given writing velocity, there also exists anabrasion-threshold-intensity I_(ATh) at and above which the LDW-glassplates are abraded or damaged on the glass surface due to excessivetemperature (>800° C.) at the laser focused spot. However, the abrasionis not a pure thermal effect, since the abrasion-threshold-intensityI_(ATh) is lower using a write beam of a shorter wavelength.

[0452] At a given writing velocity, the write-latitude is defined as thedifference I_(ATh)−I_(FE) between the abrasion-threshold-intensity andthe full-erasure-intensity. The write-latitude increases with decreasingwriting velocity and also increases with a write-beam of a longerwavelength.

[0453] At a writing velocity of 1 to 4 meter/sec the required writingenergy density for full erasure is 2 to 4 joule/cm² using a write-beamwhose wavelength in the spectral range of 488 nm to 1060 nm, providedthe optical density of the LDW-glass plate is in excess of about 0.5 atthe wavelength of the writ-beam.

[0454] The values of the writing energy density cited are based onexperimental data using write-beams having a Gaussian intensity profileat the focused laser spot. One can expect the required writing energydensity to reduce by a factor of more than 2 and the write-latitudeincreases, when a flat top intensity profile is utilized.

[0455] Multigray levels were written in LDW-glass plates using thewriting velocity or laser beam intensity or multiple retraces or acombination thereof as variable parameters.

[0456] A LDW-glass photomask with multi-gray levels is ideally suitedfor fabrication of diffractive optical elements (DOE),Micro-electro-mechanical (EM) devices, Micro-Opto-electro-mechanical(MOEM) devices. A mask for 32 phase levels of DOE is made by exposing ina laser beam writer with predetermined energy density levels accordingto a depth versus optical density calibration curve.

[0457] To transfer a multilevel resist structure of DOE into quartzthrough a dry etching process, the relative etch rate betweenphotoresist and quartz substrate can be so chosen to achieve the finalneeded etch depth 3 to 6 times that in the resist. Therefore for thefabrication of DOE in quartz a surface height variation of 1 μm in theresist results in a depth modulation in quartz of up to 6 μm.

[0458] The optical density at the wavelength of optical lithographicexposure tool, e.g., 436 nm (G line of mercury arc) is 1.4, 2.4 and 3.0for LDW-HR plates type I, type II and type III respectively. Thecorresponding optical density at 365 nm (I line) is 1.0, 1.6 and 2.0 fortype I, type II and type III plates respectively. The optical density at405 nm (II line) is 1.2, 2.0 and 2.7 for type I, type II and type IIIplates respectively. The optical density of type III plates exceeds 3.0in the spectral range of 430 nm to 615 nm. Depending on the photoresist,and its thickness requirement, one can select among type I, type II andtype III of LDW-HR plates for the required optical density at thewavelength of lithographic exposure tool. LDW-HR plates having opticaldensity of any specified value that is the same or different from thoseof type I, type II and type III plates are fabricated by controlling thevariable parameters in the darkening process using a high energy beam.The increased optical density values from type I to type II to type IIIplates are due primarily to an increased thickness of the colored glasslayer.

[0459] LDW-HR plate-type I has a larger write-latitude than the type IIplate which in turn has a better write-latitude than the type III plate.

[0460] LDW-HR plates are recommended for write-wavelengths shorter thanabout 900 nm and is also good for a write beam using CO₂ laser at 10.6μm wavelength. For write-wavelengths longer than about 750 nm, LDW-IRplates are preferred.

[0461] The optical density values at the wavelength of opticallithographic exposure tool, e.g. 436 nm are 1.2, 1.8 and 2.6 for LDW-IRplates type I, type II and type III respectively. The correspondingoptical density values at 365 nm are 1.4, 1.8 and 2.1 for type I, typeII and type III plates respectively. The optical density values at 405nm are 1.2, 1.8 and 2.4 for type I, type II and type III plates,respectively. The optical density of type II plates exceeds 3.0 in thespectral range of 570 nm to 805 nm. The optical density of type IIIplates exceeds 3.0 in the spectral range of 490 nm to 915 nm. Dependingon the wavelength of a laser writer and on the photoresist thicknessrequirement, one can select among type I, type II and type III of LDW-IRplates. LDW-IR plates having optical density values that are the same ordifferent from those of type I, type II and type III plates arefabricated by controlling the variable parameters in the darkeningprocess using a high energy beam. The increased optical density valuesfrom type I to type II to type III plates are due primarily to anincreased thickness of the colored glass layer. The type I plate has alarger write latitude than type II plate which in turn has a betterwrite-latitude than the type III plate.

[0462] Due to the effect of the erasure-threshold intensity, there islittle or no soft line-edges in the laser direct write patterns recordedin LDW-glass plates using a focused laser beam with a Gaussian intensityprofile. The recorded spot size in LDW-glass plates is substantiallysmaller than the size of the airy disc of the focused laser spot in air.Moreover, the grain size of coloring specks of silver in the LDW-glassis of atomic dimension, and LDW-glass plates have no graininess.Submicron features were recorded in LDW-glass plates using laser beamsof various visible wavelengths focused with an objective lens which hasa numerical aperture as low as 0.25.

Absorption-Phase-Shift Mask

[0463] A binary absorption HEBS-glass mask becomes a binary phase-shiftmask upon a selective wet chemical etching or a dry etching process.Starting with a HEBS-glass blank having an e-beam sensitized glass layerwhich is sufficiently thick so that the binary phase-shift mask is stillsensitive to e-beam, a second e-beam exposure produces anabsorption-phase-shift mask.

Advantages of Direct Write All-Glass Photomasks

[0464] Direct write all-glass photomask blanks includes HEBS-glassphotomask blanks and LDW-glass photomask blanks.

[0465] Direct write on HEBS-glasses or LDW-glasses create instantphotomasks, and eliminate chrome and photoresist, and their associatedprocessing chemicals. This is a zero-waste, inexpensive solution formask making. By employing the all-glass photomask blanks, photomasksmeeting specifications containing the most stringent defect levels canbe prepared consistently. Advantages gained in using all-glass photomaskblanks include the following:

[0466] 1. Photomasks can be patterned from all-glass blanks without anumber of processing steps.

[0467] 2.The advantages that can be expected from eliminating the postexposure processing steps include faster turnaround, better line widthcontrol and much lower defect densities.

[0468] 3. Defects such as intrusion, extrusion, lack of adhesion, excessmaterial/chrome spots and scratches in chrome are eliminated due to theelimination of chrome and resist as well as the associated processingsteps.

[0469] 4. No post exposure process-induced CD variation. No processinduced image quality problems (e.g., line distortion) due to resistswelling during baking.

[0470] 5. The direct write all-glass photomasks are non-reflective, andhave near zero difference in reflectivity between darkened andundarkened areas. Reflectivity is 4% which is much less than that of theanti-reflective chrome at all wavelengths.

[0471] 6. White light is a safe light for the all-glass photomask blanksenabling inspection of the mask-blanks with intense white light before,during, and after the pattern is generated.

[0472] 7. The all-glass mask is more durable than a chrome mask sincethe all-glass mask is less sensitive to surface scratching due to itsvolume effect, i.e., the masking pattern is within the glass surfacerather than on the surface.

[0473] 8. For contact printing, the all-glass masks have long life timesand more wafers produced per mask.

[0474] 9. The sensitivity of an all-glass blank is very uniformthroughout the whole blank surface. Therefore, good CD control is notlimited to the center of the mask. In contrast, chrome blanks often havethe non-uniform photoresist coating thickness near the edges of thechrome plate.

[0475] 10. Easy re-inspection of accepted masks; since there exists noscattered light from a clean all-glass photomask (without the chromefeatures which scatter light in the image plane), defects such as dustparticles, fingerprints, and scratches are readily observable/detectedin the passage of an intense light beam in a dark room. No expensiveinspection equipment are needed to reinspect used masks, or could-becontaminated masks.

[0476] 11. Unlike chrome blanks, there is no chemical waste problems.

[0477]

Applications of The Direct Write All-glass Photomask Blanks

[0478] Surfaces with three dimensional structures are required inseveral fields of micro technology. Structures with sawtooth profile(blaze) increases the efficiency of DOE. Tapered structures offer moreflexibility in the design of micro-electronics and micro mechanicalcomponents. Examples of 3D shaping using HEBS-glass gray level masksand/or LDW-glass gray level masks are:

[0479] 1. Tapered structures for microelectronics, e.g. taperedstructures in thick polyimide to realize electrical connection betweentwo metallic layers separated by the thick polyimide,

[0480] 2.Micro optical devices such as diffractive and refractive microlenses. bifocal intraocular lenses, widely asymmetric DOE, miniaturecompact disc heads. anti reflective surface, complex imaging optics,grating couples, polarization-sensitive beam splitters, spectralfilters, wavelength division multiplexers, elements for head-up andhelmet mounted display, for focal plane optical concentration andoptical efficiency enhancement, for color separation, beam shaping, andfor miniature optical scanners, microlens arrays, diffraction gratings,laser diode array collimators and correctors, aberration correction,hybrid optics, microprism arrays, micromirror arrays and Bragg gratings.

[0481] 3. Integrated optical components, two dimensional fanoutgratings, optical interconnect, signal switching, fiber pig tailing, DOEto couple light from a laser into a fiber,

[0482] 4. Micro-electro-mechanical (EM) devises for sensors andactuators in automotive, machine tools, robotics and medical infusion,also devices for applications in micro valves, inertial micro sensors,micro machined RF switches, GPS component miniaturization, and a host ofother sensors and actuators for applications to space, air, land, andsea vehicles, as well as industrial, biotechnology and future consumerelectronics,

[0483] 5. Micro-opto-electro mechanical (MOEM) devices such as laserscanners, optical shutters, dynamic micro mirrors, optical choppers andoptical switches.

[0484] In the description which follows like elements are markedthroughout the specification and drawing with the same referencenumerals, respectively. Drawing figures showing actual physical elementsare not intended to be to scale.

[0485] Several types of micro-elements are required to be of a threedimensional configuration which includes a variable surface contour orgeometry and which may be symmetric or non-symmetric. Micro-opticdevices such as micro-lenses, wave guides and computer generatedholograms, for example, often require a geometry which is preferably acontinuously curved surface or which has a profile of continuouslyvarying depth from a reference point. Examples are diffractive opticalelements such as spherical micro-lenses, Fresnel lenses and certainoptical waveguide and coupling devices. The fabrication of such elementsmay be carried out, generally, using methodology similar to that usedfor the fabrication of very large scale integrated circuits (VLSI)wherein a photoresist material is placed on a substrate and is etched toproduce a replica of the micro-element, preferably to a finely detailedand precise geometry. This precision geometry is particularly importantfor micro-optic devices and micro-machines, for example.

[0486] Gray scale masks (also known as gray “level” masks) have beendeveloped, as mentioned hereinabove, to provide the necessary surfacecontours of micro-elements, including micro-lens devices, to replace themulti-step binary fabrication methods for these devices. However, theshortcomings of prior art gray scale masks mentioned hereinabove haveinhibited the development of a method for volume production ofmicro-optic elements and other micro-elements. In accordance with thisinvention it has been discovered that an improved gray scale masksuitable for use in the fabrication of precise, highly efficientmicro-optic elements can be provided using, as the mask structure, glassplates which have been composed to be sensitive to controlled electronbeams to generate a darkened image in the glass having a preciseconfiguration and having a substantially continuously varying lighttransmissivity capability over a pre-determined area.

[0487] The present invention contemplates the provision of a gray scalemask comprising a structure formed of a base glass such as a lowexpansion zinc-boro-silicate glass or so-called white crown glass. Thebase glass composition also contains alkali to facilitate an ionexchange reaction which achieves the sensitivity of the glasscomposition to high energy beams. After ion exchange the glass materialbecomes alkali-free as a result of the ion exchange process, which istypically carried out in an acidic aqueous solution at temperaturesabove 320. degree. C. The base glass composition comprises silica, metaloxides, nitrates, halides and photo inhibitors. Typically, TiO.sub.2,Nb.sub.2 O.sub.5 or Y.sub.2 O.sub.3 are used as photo inhibitors. Thephoto inhibitors are used to dope silver-alkali-halide complex crystals,for example. The (AgX).sub.m (MX).sub.n complex crystals are beamsensitive and the doping process increases the energy band gap of theotherwise photosensitive material.

[0488] The beam sensitive glass used in the present invention may be ofa type may be such as described in U.S. Pat. No. 5,078,771 issued Jan.7, 1992 to Che-Kuang Wu, which is incorporated by reference herein.Other glasses which are beam sensitive and which may be used tofabricate a gray level mask in accordance with the invention aredescribed in U.S. Pat. No. 5,114,813 issued May 19, 1992 and U.S. Pat.No. 5,145,757 issued Sep. 8, 1992, both to Steven W. Smoot, et al.,which are also incorporated herein by reference. However, the inventionis not necessarily limited to the particular glass compositionsdescribed hereinabove for fabrication of the gray scale masks and othermaterials which are darkenable in different degrees in accordance withthe invention may be used.

[0489] Accordingly, a gray scale mask in accordance with the presentinvention may be fabricated from a glass structure or similar materialcomprising a relatively thin plate which, after being drawn, ground andpolished is treated in such a way that at least one surface of the plate(or a similar glass article) becomes effective to render the surfacedarkenable upon exposure to electron beam radiation over at least aportion of the surface and wherein the plate or article is preferablysubstantially not alterable by or sensitive to actinic radiation.Preferably, the gray scale mask article is exposed to a high energybeam, such as an electron beam, preferably at an acceleration voltage ofgreater than about 20 kV (kilovolts) whereby the necessary change intransmissivity or optical density of the article is such that a grayscale mask can be produced while maintaining good resolution of theimage produced on the glass.

[0490] Referring to FIGS. 21 and 22, and by way of example, diagrams ofthe optical density or optical transmissivity of a gray scale mask inaccordance with the invention are illustrated showing the variation inoptical density for light of wavelengths between about 350 nm(nanometers) and 550 nm for electron beam acceleration voltages of 20 kV(FIG. 21) and 30 kV (FIG. 22) for electron charge densities or “dosages”ranging from 0 to 367 .mu.C/cm.sup.2 (micro-coulombs per centimetersquared). A writing current of about 25 nA (nanoamperes) was used inobtaining the data for FIGS. 21 and 22.

[0491] A gray scale mask comprising a glass article of a composition inaccordance with the teachings of U.S. Pat. No. 5,078,771 can be producedusing a commercially available electron beam writing device. Thesedevices can be controlled to expose the glass article, such as a plate,to an electron beam to generate images of varying optical densitywherein the image is generated on a grid having grid spacings of about0.1 mm, for example. The grid spacings may be smaller if desired but thewriting time is increased accordingly. Larger grid spacings will tend toreduce image resolution. The lower of the two acceleration voltages usedto generate the data in FIGS. 21 and 22 is preferable to minimize thespreading of the darkened spacings on the grid by the electron beam.Since the size of the interaction volume of the electron beam with theglass material depends on the energy of the incident beam the lowestacceleration voltage which still achieves a high enough penetrationdepth for sufficient optical density is preferred. An accelerationvoltage of 20 kV produces enough penetration in the glass material of agray scale mask as described herein without extending the electrontrajectories unnecessarily in a way which would result in the loss ofimage resolution. The operating parameters of the beam writer or asimilar device may be varied with the particular beam sensitive materialbeing used to fabricate the gray scale mask and the values given hereinare for an exemplary embodiment of the invention.

[0492] Preparation of a glass plate utilizing the electronbeam-sensitive glass for generating the pre-determined gray scaledarkened areas preferably includes depositing a so-called groundinglayer of material on the surface of the glass. The purpose of this layeris to avoid local electrical charging of the mask plate during theelectron beam writing process. A grounding layer in the form of a layerof chromium of a thickness of about 10.0 nm may be applied to the glassplate surface adjacent to the ion exchanged surface layer of the glasscontaining the high concentration of silver ions. The chromium groundinglayer may be removed by wet etching after the mask plate is darkened tothe predetermined gray level pattern desired.

[0493] In the fabrication of a diffractive optical element as well asother micro-elements using a photoresist material on the surface of asubstrate and an etching process to develop the photoresist and thesubstrate, a correlation must be obtained between the electron chargedensity or dosage (in coulombs per centimeter squared, for example)which will be applied to the gray scale mask and the correspondingthickness of penetration of the photoresist during the resultantexposure of the photoresist through the gray level mask. FIG. 23 shows,by way of example, a calibration curve for depth of penetration in aphotoresist, such as a type OeBR-514 photoresist available fromOlin-Ciba-Geigy Corporation, for example, compared with electron beamcharge density applied to the mask grid spacings, respectively. In otherwords if a depth contour in the substrate such as a diffractive opticalelement is to be correlated with the thickness of a photoresist which isto be altered by exposure through the gray level mask, then acorresponding degree of darkening of the mask must be achieved and theelectron dosage which will achieve this darkening can be correlateddirectly with the degree of penetration or exposure of the photoresist.For the sake of discussion herein it will be assumed that, if a largeamount of exposure light is transmitted through a particular mask to thephotoresist, then the height of the processed photoresist is limited andthe thickness of the micro-element produced in the etching process iscorrespondingly small. If the amount of light transmitted through aparticular mask opening is small, then the height of the processedphotoresist is large and the corresponding thickness or height of theprocessed substrate article is also correspondingly large. Photoresistmaterials which, upon exposure to varying light intensities, respond inthe opposite manner upon processing, may, of course, be used inconjunction with the method of the present invention.

[0494] In designing a surface profile for a particular diffractiveoptical element, a particular number of evenly spaced depth levels maybe selected. For example, for a photoresist thickness of about 350 nm,thirty-two depth levels of penetration of light which will alter thecharacteristics of the photoresist may be selected and these differentdepth levels may then be correlated with a particular electron beamdosage to cause the appropriate darkening of the gray level mask. Inother words, thirty-two different gray levels are generated.

[0495] For the particular gray level mask discussed herein, anacceleration voltage for the electron beam of 20 kV may be selected, soas to avoid substantial exposure time of the beam at each grid spacing.For the production of diffractive optical elements such as sphericallenses having a focal length of about 4.4 mm and a lens size of about100 m.times. 100 m a grid spacing of about 0.1 m may be selected, forexample. A photoresist having a thickness of about 350-500 nm willproduce a depth in the micro-element substrate in the range of three tosix times the depth of the photoresist so that, for example, thesubstrate profile may have a total depth of up to about 2100 nm, by wayof example. Again, by way of example, the design of the surface profileis spaced out over thirty-two evenly spaced depth levels over a 350 nmthickness range of the photoresist. With a grid spacing of 0.1 m thedifferent depth levels may then be written into a computer program forcontrolling the electron beam writer in a way wherein the programcontrols the writer to dwell for a predetermined period of time at eachgrid spacing.

[0496]FIG. 24 illustrates a cross sectional profile for a generallycircular lens 9 having a hub portion 10, concentric circumferential lenssurfaces 12 a, 12 b, 12 c and 12 d and corresponding concentriccontoured lens surfaces 14 a, 14 b, 14 c and 14 d. These lens surfacesmay be repeated at selected radii from the hub portion 10 in accordancewith known practices for spherical or Fresnel lens design, for example.Referring to FIG. 25B, a portion of the cross-sectional profile of thelens 9 is shown on a processed photoresist layer 16 disposed on asubstrate 18 of a suitable light transmissive material, such as quartzglass, silica glass or germanium to be used as the lens itself. Quartzglass is used in an example described hereinbelow.

[0497] In FIG. 25B, the layer of photoresist 16 is shown with thecontour or profile of the lens 9 formed therein for the sake of clarity.Accordingly, a portion of the hub 10 of lens 9 is indicated at 10 r, thecontoured lens surface 14 a is indicated at 14 ar, the circumferentiallens surface 12 a is indicated at surface 12 ar and the contouredsurface 14 b is indicated at 14 br, for example.

[0498]FIG. 25A is intended to be read in conjunction with FIG. 25B andshows a portion of a grid 20 having the spacings mentioned above andshowing, for example, grid spacings 21, of equal area, that is, squaresof 0.1 .mu.m, exemplary ones of which are shown darkened to varyingdegrees to provide the thirty-two depth or phase levels in thephotoresist 16 and eventually in the lens 9. Of course, the grid 20exists only in the micro-processor which controls the electron beamwriter and the spacing of the writing mechanism as it moves from onespacing 21 on the grid to the next spacing and the electron beam is thengenerated to darken the spacings, accordingly. Representative darkenedgrid spacings are indicated at 22 defining the edge of thecircumferential surface 12 a, for example. Representative grid spacings21 which have been slightly darkened, as indicated at 24, show thecontour of the periphery of hub portion 10 of the lens as represented onthe contour of the etched photoresist 16. Darkened grid spacingsindicated at 25 define the edge or juncture of surfaces 12 br and 14 br,for example.

[0499] Accordingly, for a particular photoresist material, the thicknessof the photoresist which is to remain after exposure and etching may becorrelated with the electron beam dosage required to darken a highenergy beam-sensitive glass of the type described herein and the contouror profile of a micro-optic element or other micro-element may becorrelated with the dosages or beam charge density required for eachsubdivided space in a grid. The electron beam writing device may becontrolled to index to each of the spaces 21 in the grid and apply apre-determined electron beam dosage to that space corresponding to thedegree of darkening of the gray level mask desired. Of course, the sizeof the grid spaces may be varied, the acceleration voltage may be variedand the electron beam charge density-may be varied depending on thecharacteristics of the particular mask material and the photoresistmaterial being used.

[0500] Electron beam dosing of the gray scale mask plate at each of thegrid spacings 21 will darken the glass across the grid to produce thegray levels desired. After removal of the aforementioned electricalcharge grounding layer no further treatment of the gray scale mask isnecessary, the gray levels are visible and repairs or additional imageconfigurations or other features may be provided by additional writingoperations with the electron beam writer.

[0501] Fabrication of micro-elements, such as diffractive opticalelements, may then be carried out using a gray scale mask fabricated inaccordance with the description hereinabove. For example, a gray scalemask 30 is shown in FIG. 26 comprising a glass plate formed of a glassof the type described above and in the patents referenced herein andwhich has been exposed to an electron beam writer to generate gray levelimages of an array of generally spherical micro-lenses, as indicated bythe images 31 in FIG. 26. These images are in the surface layer 30 afacing surface 16 a of the photoresist layer 16. The gray scale mask 30may then be brought into contact or close proximity with surface 16 a ofphotoresist 16 which is disposed on a quartz glass plate 18.

[0502] Equipment used in the production of micro-electronic devices byexposure of photomasks to photoresist coated substrates may be used toproduce diffractive optical elements in accordance with this invention.For example, the gray scale mask 30 may be placed in contact with orclose proximity to the layer of photoresist 16 in a commerciallyavailable aligner and the photoresist of the type mentioned above isthen exposed to light in a range of wavelengths of 327 nm-400 nm, forexample. The mask 30 may also be disposed remote from the photoresistand the mask image projected onto the photoresist using an opticalimaging or scanning system. Accordingly, photo reduction(demagnification) or photo enlargement (magnification) of the image inthe gray scale mask may be carried out on photoresist, if desired.

[0503] Preparation of the substrate and photoresist, using a photoresistof the type mentioned hereinabove (OeBr-514) or another ultravioletcurable polymer may be carried out by spinning a layer of photoresistonto the substrate at a speed of about 4,000 rpm, for about 30 seconds,for example. A 0.8 .mu.m thick layer applied to a quartz glass substratemay be obtained and, the photoresist coated substrate baked for about 30minutes at 90. degree. C. prior to exposure of the resist through thegray level mask.

[0504] After exposure of the photoresist 16 with the gray scale mask 30,development of the photoresist may be carried out by post-baking theresist for a predetermined period and at a relatively low temperature soas to avoid reflow of the resist during the post-baking procedure whichmight result in a degraded profile of the micro-element. Alternatively,the photoresist may be developed in a metal ion-free developer such as atype made by Shipley Corporation. The photoresist-coated substrate maythen be subjected to a conventional etching process such as an ion beammilling procedure. A Veeco Instruments Microtech 301 -type millingsystem may be used, for example.

[0505] The ion milling system may be modified to accommodate theintroduction of reactive gasses to provide chemically assisted ion beametching. Chemically assisted ion beam etching is advantageous because itallows for the accurate control of the energy, number and incidenceangle of ions during the milling process. Moreover, the amount ofreleased reactive gas can be chosen freely which allows for control ofthe q-factor. The q-factor is defined as the substrate etch rate to theresist etch rate. Varying the q-factor provides for varying the featuredepth in the final micro-element to fit a specific application. Forexample, in the fabrication of diffractive optical elements, the featuredepth in the final configuration will be dictated by the specificapplication, wave length of light to be transmitted, the refractiveindex of the substrate material, the refractive index of the surroundingenvironment, or dimensional constraints on the substrate structure.Variation in the amount of reactive gas, such as CHF.sub.3, introducedinto the etching system will allow a change in the q-factor ranging from1.8 to 4.3, for example. A higher q-factor is usually necessary toachieve a high etch depth required for elements transmitting longerwave-length light and also allows the reduction in the resist thicknesswhich will result in enhanced resolution. Lower q-factors may be usefulin achieving low feature depth and high accuracy needed for reflectiontype optical elements.

[0506]FIG. 27 illustrates a portion of the actual profile of amicro-spherical lens substantially like lens 9 and fabricated inaccordance with the present invention wherein an overall height of thelens profile in the range of about 2000-2100 nm was achieved. Thepreviously described technique was used to produce a 2.times.2 array ofspherical lenses having an f number of nineteen and a focal length of 96mm. A gray scale mask was fabricated of a high energy beam sensitiveglass plate having a thickness of approximately 90 mils. The mask wasexposed in a Cambridge Model EBMF 10.5 Electron Beam Writer which wascontrolled in accordance with aforedescribed procedure to producethirty-two discrete levels of darkening of the mask in a predeterminedpattern on 0.1 .mu.m spacing. The electron beam writer was controlled bya computer-aided design program developed for the University ofCalifornia at San Diego to facilitate data generation necessary fordirect write procedures with the electron beam writer. Electron beamcharge density level for each depth or phase level in the final etchedelement profile can be included in data files used to operate theelectron beam writer. This may be carried out by changing the writingfrequency for different areas of the produced micro-element. Substratematerial for the diffractive optical elements was fused silica. Thediffractive optical element was, in particular, designed for anoperational wavelength of 830 nm. The optical efficiency of the lensproduced showed a 94% efficiency which is comparable to an efficiencymeasurement taken for a substantially identical lens fabricated bydirect write methods.

[0507] A 10.times.10 array of spherical lenses of 100 mm by 100 mm size,an f number of 3.10 and a focal length of 4.4 mm was also fabricatedusing the above mentioned process for fabrication of the gray scale maskand the subsequent fabrication of the optical elements utilizingchemically assisted ion beam etching to transfer the resist profile intoa quartz glass substrate.

[0508] As mentioned previously, a gray scale mask in accordance with thepresent invention may be advantageously used for mass production ofdiffractive optical elements, computer generated holograms and othermicro-elements in a step and repeat fabrication system. In particular, arelatively large substrate member, suitably coated with photoresist maybe exposed through a gray scale mask, such as the mask 30 in acommercially available aligner of a demagnification type or a contacttype wherein the geometry of plural diffractive optical elements may beimprinted on the photoresist, the substrate member may be moved relativeto the gray scale mask and the exposure step repeated so that a largearray of micro-elements is imprinted on the photoresist step by step.This relatively large array of micro-elements may then be fabricated ina batch by a chemically assisted ion beam etching process, as describedabove, to transfer the geometry of the-micro-elements in the photoresistto the substrate member. Step and repeat or so-called stepper processesmay, thus, be carried out with gray scale masks in accordance with theinvention. Accordingly, the manufacture of various types ofmicro-elements as described herein, may be more efficiently andeconomically carried out.

[0509] Although preferred embodiments of the present invention have beendescribed in detail herein, those skilled in the art will recognize thatvarious substitutions and modifications may be made to the inventionwithout departing from the scope and spirit of the appended claims.

What is claimed is:
 1. A method of fabricating a three-dimensionalmicro-optic lens on a substrate selected from a group consisting ofquartz glass, silicate glass, germanium and an optically transmissivematerial coated with a photoresist layer, comprising: providing a grayscale mask having a body portion and a surface layer formed thereonwhich is responsive to electron beam radiation to change the opticaldensity of the surface layer; exposing the mask to an electron beam ofselected charge density over a grid of discrete locations on the mask toprovide a predetermined gray scale pattern of continuously varyingoptical transmissivity on the mask; exposing the photoresist layer toradiation transmitted through the mask; and removing material from thephotoresist layer and the substrate to provide a predetermined varyingthickness of the substrate as determined by the gray scale patterns onthe mask.
 2. The method set forth in claim 1 including the step of:generating said electron beam with a current of at least about 25 nA. 3.The method set forth in claim 1 including the step of: applying anelectrically conductive coating to the mask prior to exposing the maskto said electron beam and removing said coating from the mask afterexposing the mask to said electron beam.
 4. The method set forth inclaim 1 including the step of: comparing a thickness of said photoresistlayer which may be exposed to radiation with a corresponding electronbeam charge density value required to darken said layer of the mask toprovide a predetermined depth level in said substrate; and exposing themask to said electron beam at a preselected charge density correspondingto the desired thickness of exposure of said photoresist layer.
 5. Amethod for producing various depth levels in a layer of photoresistmaterial including the steps of: exposing a layer of photoresistmaterial to radiation through a gray scale mask having areas ofcontinuously varying transmissivity; removing photoresist material fromsaid photoresist layer to depth in said photoresist layer at apredetermined position thereon corresponding to a predeterminedtransmissivity of said gray scale Mask at a corresponding predeterminedposition on said gray scale mask; and providing said gray scale mask asa glass article comprising a body portion and an integral ion exchangedsurface layer which, upon exposure to a high energy electron beam,becomes darkened and is substantially insensitive to actinic radiation.6. The method set forth in claim 5 including the step of: exposing saidgray scale mask to selected discrete charge densities of electron beamradiation over a grid of preselected grid spacings and varying theelectron beam charge density from one spacing to the next in accordancewith a predetermined depth level desired to be produced in saidphotoresist layer.
 7. The method set forth in claim 5 including the stepof: comparing a thickness of said photoresist layer which may be exposedto radiation with a corresponding electron beam charge density valuerequired to darken said gray scale mask to provide a predetermined depthlevel in said photoresist layer; and exposing said gray scale mask tosaid electron beam at a preselected charge density corresponding to thedesired thickness of exposure of said photoresist layer.
 8. The methodset forth in claim 5 including the step of: selectively darkening asurface layer of said gray scale mask by generating an electron beam atdiscrete, predetermined positions thereon and at an acceleration voltageof at least about 20 kV.
 9. A method of fabricating a three-dimensionalmicro-element on a substrate to various depth levels comprising one ofdiscrete depth levels and a continuous depth profile through aphotoresist layer, comprising the steps of: exposing said photoresistlayer to radiation transmitted through a gray scale mask having a grayscale pattern thereon comprising image areas having a continuouslyvarying transmissivity corresponding to a depth of material to beremoved from said substrate to provide said element; removing materialfrom said photoresist layer and said substrate in a predeterminedpattern as determined by said gray scale pattern on said mask; providingsaid gray scale mask characterized as a glass article comprising a bodyportion and an integral radiation absorbing surface layer which issubstantially insensitive to actinic radiation; and providing said glassarticle with said ion exchanged surface layer having Ag+ ions therein,and/or silver halide containing and/or Ag.sub.2 O containing and/or Ag+ion containing micro-crystals and/or micro-phases therein.
 10. Themethod set forth in claim 9 including the step of: exposing the mask toan electron beam at a predetermined dosage corresponding to a degree ofdarkening of the mask required to produce a predetermined depth level insaid photoresist layer.
 11. The method set forth in claim 10 includingthe step of: darkening the mask by generating an electron beam at anacceleration voltage in the range of 20 kV to 30 kV.
 12. The method setforth in claim 10 including the step of: exposing the mask to anelectron beam charge density of 0 mC/cm.sup.2 to about 400 mC/cm.sup.b2.
 13. The method set forth in claim 10 including the step of:generating said electron beam with a current of at least about 25 nA.14. The method set forth in claim 10 including the step of: applying anelectrically conductive coating to the mask prior to exposing the maskto said electron beam.
 15. The method set forth in claim 14 includingthe step of: removing said coating from the mask after exposing the maskto said electron beam.
 16. The method set forth in claim 10 includingthe step of: comparing a thickness of said photoresist layer which maybe exposed to radiation with a corresponding electron beam chargedensity value required to darken the mask to provide a predetermineddepth level in said substrate; and exposing the mask to said electronbeam at a preselected charge density corresponding to the desiredthickness of exposure of said photoresist layer.
 17. The method setforth in claim 16 including the step of: exposing the mask to selecteddiscrete charge densities of electron beam radiation over a grid ofpreselected grid spacings and varying the electron beam charge densityfrom one spacing to the next in accordance with a predetermined depthlevel desired to be produced in said substrate.
 18. A method offabricating a three-dimensional diffractive optical element withinphotoresist which is coated on a substrate selected from a groupconsisting of quartz glass, silicate glass, germanium and an opticallytransmissive material comprising: providing a HEBS glass photomask blankhaving a body portion and a surface layer formed thereon which isresponsive to electron beam radiation to change the optical density ofthe surface layer; exposing the HEBS glass photomask blank to anelectron beam of selected charge density over a grid of discretelocations on the photomask blank to provide a predetermined gray scalepattern of varying optical transmissivity on the photomask blank toproduce a gray scale mask; exposing the photoresist layer to radiationtransmitted through the mask; and removing material from the photoresistlayer to provide a predetermined varying thickness of the photoresistlayer as determined by the gray scale patterns on the gray scale mask toproduce the three-dimensional diffractive optical element.